When you ride a bike on a trainer the bike is fixed in place, can not move side to side. When you ride on the road the bike can move and the wheels absorb quite a bit of flex you put of the force you put the bike through while riding. So when you put the bike on a trainer it is mount via the axle in the back wheel so all that force you are using to push the bike side to side and downwards is put directly on rear triangle and your BB of the frame which has damaged frames. Now in reality would this be a problem for 99% of us out their probably not, but why put extra stress on you're frame if you don't have to. When you ride rollers the bike can move a bit side to side and the wheels still are there to absorb alot of the force.