Fantastische Hure said:
They let that happen against Kid Boo though and like I said Vegeta told Goku he can take Boo as a SSJ3 but never said the same against Super Boo because he knew Goku was right and they were about to get destroyed.
Besides I'd rather try my best at fighting someone equally (especially since this is Goku who loves a challenging fight) than the possibility of being permanently fused with someone for the rest of my life.
First of all my argument is not about Goku having a power equal Boohan so let's throw it out of way.
Also it was already stated why Fusion Dance shouldn't be use in this situation.
Elder Kaioshin himself agreed that FD is useless in this situation not even Goku tried to contradict him by telling that he could just grab Gohan out there with IT and teach him how to fuse.
Another reason why Goku was willing to fuse with Vegeta forever so that Vegeta doesn't have to go back on the after-life once Boo is dead.