The Killswitch (Protobomb) is a nice trademark move, but with the FU he can stall and get the crowd going, knowing what's coming without him having to call it out. When he's on his shoulders, the crowd starts to cheer; with the Killswitch it's not a quick enough move like the RKO, and make it seem out of nowhere, and it doesn't give the crowd a chance to cheer for what's coming up anyway. The FU does.
I'm glad he's dropping down with the opponent. Knees or BUTT, it doesn't matter. Makes it look 5 times better.