A few notes to begin:
1) I am not a Brock nuthugger, but I don't mind reading about him and watching him. I consider him a true superstar in sport, media and entertainment
2) I am a Cain fan and was cheering for Cain to win the title
I like to put my conclusions first, as a good management consultant should, and then back them up with reasoning. Inductive, pyramid style logic.
Conclusion: Brock Lesnar doing a one-off WWE show would potentially be a plus for Brock, UFC and MMA as a whole
1) Huge profile around the event, raising Brock Lesnar's profile even further (it of course fades a little whenever he's inactive). When Ken Shamrock was in WWE, numerous mentions were made of his status as a UFC fighter... free promotion for UFC, and perhaps drive interest among WWE fans to watch Brock's next fight in UFC. MMA as a whole benefits when its stars gain any additional exposure outside the MMA promotions. It might make sense for Dana to insist that Brock thanks the UFC on TV for freeing him up? (not sure on whether that last bit is worthwhile)
2) Dana does WWE/Vince a favor, and you never know when a return favor is needed
3) Brock earns huge $ from the event. Dana could demand a cut of that in order to free up Brock for the WWE event, or else extract some other concession from Brock (e.g. regarding bonuses in future or contract re-negotiation).
4) Brock/Dana should insist that the worked fight has Brock winning. Sure, it's all fake, but the UFC has got to want "real" fighter beating "fake" fighter. I don't see that a small return to WWE hurts UFC's image as a legit fighting organisation - after all, Brock came from WWE in the first place. Any attempt to screwjob Brock, I think Brock can "no-sell" to use the wrestling terms (which I find wanky, but relevant here).
5) As a fan, I love entertainment and used to watch WWF as a kid. I would have fun watching Brock on WWE, and it certainly won't reduce my willingness to watch his next UFC fight.
The same kind of thing applies to movies. I think Randy and Rampage being in movies helped UFC, given that they returned to fight in UFC afterwards. Even if they didn't, having ex-UFC fighters in mainstream media/entertainment should help UFC's profile.