They're incredibly fast.Alas- how do you even win games with bikes?! They can't get to obscured objectives, and they can't seem to kill anything! Yet people tell me that they're a powerful army?!
You don't have to out CC a horde army when you can move 12" a turn and then blaze away with bolter rounds + attack bike guns, plus you can get flamers in the squad which tend to help. all in all a bike army tends to be a bit more flexible, and if you've the last turn you can just crash your bike squad into an objective and as long as you don't lose the whole squad to difficult terrain tests then it's contested!