Rug wrote:
Bikes have better stats than Tacticals and Assault Marines and are cheaper, I'm sometimes at a loss as to why people bother with Tacs and Assault Marines!
I agree.
Bikes are definitely better than Assault marines in every respect. Coupled with Land-speeders they can make a very powerful harassment force. Devastators and Land-speeders are good for laying BMs and supporting assaults, and Terminators excel in every area except mobility, and that is what THawks are for

Tacticals perform all the above less well than the relevant specialists. Indeed, two formations of Assault marines with a commander are better than a Tactical formation with a leader (although this costs 75 points less) because of their innate mobility and the second activation.
Arguably, Devastators are the most utilitarian because they can be upgraded to work well in many tactical situations, though Bikes run a close second because of their mobility.
The point about Bikes is that they can be carried in air-transport, have a better assault than all other marines except Terminators, have innate mobility that cannot be shot and they can still lay a BM after doubling so are also a viable support formation.
I suspect the reason they are not used as much is actually down to the numbers available on sprues . . . .