This morning I fired off a quick email to my friend fester about insurgent attacks on roads and bridges, which he has expanded into an important post. The key point is that cutting our transportation options makes it much harder for US forces to move around Iraq freely while expanding the ability of insurgents to plan ambushes. As difficult a job as our troops have today, every blown bridge magnifies the difficulty by making it easier for insurgents to predict where convoys will drive while cutting our ability to respond to emergencies. We really, really do not want to end up defending the one good bridge left between Kuwait and our forward operating bases.
Also read Larry Johnson at Kos, who has more or less the same concern.