If there's a road between the spawn and the map room, then it's advantageous to select a truck with troops and get to the objective very quickly - as soon as the target is ready.
Of course, it's also a good idea for Mr. Defender to lie in ambush.
We could also do with more roads - but at the moment they're just used as tracks for those rare (and VERY fast!) convoys. BTW, convoys I've seen are pretty slow, the ones in AH seem to speed along. Far too fast.