my big beef with bikes is that there are too many of them to fit anything else inside a thunderhawk with them, but too few to be really effective....
If you can manage it, I've found them best as a T3 air-assault formation in a thunderhawk, refrain from engaging and fly the thawk around shooting stuff up, then on T3 it stands a fair chance of nabbing an objective with a timely air-assault, especially if they have a chaplain