Two points, if I may...
First, it been illegal in the modern era to cause unnecessary suffering, and it's worth remembering that one of the first arms control treaties banned dum-dum bullets in the late 19th Century.
The second point is that weapons have to be discriminating - it's never legal to target civilians, though some level of collateral damage may be inevitable.
Landmines can cause unncessary suffering and won't always be discriminating. And unexploded cluster submunitions will also be problematic.
However.... the legality or illegality depends how you use them. And therefore they shouldn't be made illegal per se.
The OVERIDING point is this: we are fighting a war against a determined, resourceful, and in some cases, fanatical, foe. But we are ultimately saying that our conception of a better Iraq/Afghanistan is better than our opponents, and part of this is upholding higher ideals - including international law. The moral high ground is as important as actual high ground, and defending it also costs lives.
Let's engage in the debate and win it on the quality of our argument.
S41