There is a reason Urahara Kisuke never releases his Bankai.
Kurosaki Ichigo, personally, believes it's because Urahara doesn't actually have Bankai; that he is a little like Zaraki, simply too talented—too dangerous—to have been refused a Captaincy even when he lacks what should be an integral part of the position. For all that the man says he achieved it in three days, for all that Ichigo has used the same method Urahara reportedly used to gain Bankai, a part of Ichigo thinks he's making it all up – that he lost to his Zanpakutō and has been lying about it for over a century. It would not be the first time, after all.
Tsukabishi Tessai doesn't actually care – Kido is every bit as dangerous as a released Zanpakutō, even in Bankai, and he knows full well Urahara's talents in that particular area of the Shinigami arts. Between his Kido, his Hakuda, his Hohō, his Zanjutsu and his Shikai—not to mention his genius—Tessai has never bothered to wonder why he's never seen his friend's Bankai. Urahara has so many other options available to him that he's probably just never felt the need. Not even against Aizen.
Others think it may because it's sacrificial, that his Bankai can harm even its wielder, like Rukia's, or Byakuya's – and everyone knows that Urahara is inherently selfish. That he would never risk himself for the greater good. It should come as no surprise that Suì-Fēng is amongst this number.
Urahara often wonders how Suì-Fēng would feel if she knew exactly how close to the truth she was. Benihime's final release—he dare not even think her name, not now, not ever—is indeed sacrificial. The only problem, however, is that it is not sacrificial for him.
In her Shikai form, Urahara often finds himself comparing Benihime to a rose; all sharp thorns and vibrant beauty. She steals energy from the world around her, and shapes it into screaming death; she is a princess, after all, and princesses have no need to do anything but take. She is breathtakingly arrogant, carnal as sin, and when she laughs, it is in the way madness does. Benihime is everything Urahara refuses to be – everything he could be, but won't. Chaos, destruction, rage; she is a broken-glass-and-bleeding mirror of the man Urahara has never allowed himself to become.
This, he thinks, is why he loves her. It might be the ultimate act of narcissism—to fall in love with your own soul—but Urahara has never found that he particularly cares. He has loved her since before he knew her name, and he has come to accept this is as simple truth. Urahara is a scientist – he does not argue with the facts, merely tries to understand what they mean.
He often wonders why other people do not—cannot—do the same. There are very few things that are a mystery to Urahara Kisuke, a man who even Aizen Sōsuke respects for his intelligence, but this is one of them. If something is true, it is true. It does not matter if you want it to be false. The universe has never expressed a particular tendency to be swayed by the opinions of others.
Yoruichi once called his outlook cruel – that it allowed for no hope, no emotion, nothing but raw, brutal truth. He'd laughed, briefly, then asked her if she thought false hope was kinder. His oldest friend had no answer. Urahara often tries to hide how much Benihime influences his thoughts. Sometimes he does not succeed. They are not two sides of the same coin, after all; a better metaphor would be that they are two edges of the same blade. Even he is not always sure which one of them is doing the cutting.
It reminds him of what he once said to Ichigo, about fear, about resolve and reflections in a sword. Urahara's fear has never been reflected in his sword. No, it's entirely the opposite – his sword is a reflection of his fear. And his resolve to master it.
They say Ryūjin Jakka is the most powerful Zanpakutō in Soul Society. Urahara finds this mildly amusing, especially in light of Aizen's betrayal. It is true that the Captain-Commander's blade has by far the most raw, offensive might of any Zanpakutō in history; even in its Shikai form, Urahara would not be surprised to find out it could level cities and burn mountains to dust. But, for all that, even Yamamoto was completely fooled by Kyōka Suigetsu – Aizen ran rampant for over a century without a single person seeing through his illusions.
There is a difference between strength and power, and Urahara has never seen it so starkly illustrated; the Captain-Commander ruled the Gotei 13 through the strength of Ryūjin Jakka, and Aizen ruled him through the power of Kyōka Suigetsu.
It is a similar story with Benihime's Bankai. While it does not have anywhere near the same, almost unholy strength of Yamamoto's Shikai, in many ways it is infinitely more dangerous.
After all, there is a reason Urahara never releases his Bankai.
The last time he did, it cost his best friend part of her soul.
Yoruichi, too, never releases her Bankai, or even her Shikai. Most presume that this is because she does not need them – and this is true. It is simply not the whole truth. Her Shikai is indeed weaker than her Shunkō. Her Bankai, however, is not. But a fundamental rule of Zanpakutō—with one notable exception—is that a broken Bankai can never be repaired, and Yoruichi's was shattered long ago.
Benihime is a jealous lover, all arrogance and avarice, and she has always hated Yoruichi. It should have come as no surprise to Urahara, when they agreed to spar with one another using their respective Bankai once he achieved the final release, that Benihime would take her chance to punish the Shihōin noble – to show her who the real princess was.
If Benihime's Shikai is a rose, her Bankai is a dagger; a dagger that has just been torn from an enemy's throat, soaked in blood like a lover's blush. No longer does she simply steal energy from the world around her – in her final release, she is above such petty thievery. No, when Benihime is in Bankai, she is a princess amongst peasants, accepting the tributes they offer her for merely existing. It is not her fault, after all, if those tributes happen to be their souls.
This is the true horror of Urahara Kisuke's Bankai. In her final release, Benihime feeds on the Reiryoku of anyone but her wielder in order to overpower her Shikai abilities. An unprotected soul is erased from existence in a few, scant moments, an average Shinigami in perhaps thirty seconds—in agonizing pain all the while—and seated officers and Lieutenants in a couple of minutes. Even Captain-class Shinigami are not immune to her power – the difference is that they have enough strength to potentially sate her appetite, especially if they release their Bankai.
That, after all, was how Yoruichi lost the ability to release hers. It was only through sheer desperation that Urahara managed to seal Benihime before she went any further.
Ryūjin Jakka could burn the entire world. Kyōka Suigetsu could fool it. But Benihime could consume it, and as far as Urahara is concerned, that is something far more monstrous. At least if someone is killed by the other two, their soul has a chance to reach the Rukongai. At the hands of his Crimson Princess, there is only oblivion.
Urahara has never bothered to wonder what that says about him.
He already knows.
Because Urahara's Bankai will be a shadow of his soul, and there is nothing more dangerous than a scientist with an unquenchable thirst for knowledge.