Well, I think the landmine treaty was successful in shrinking the market for landmines, by removing many producers and stockpiles from the marketplace, and placing pressure on the holdouts to be more judicious in their use and export of mines. If the major powers get into a shooting war, landmines will probably be a minor concern, but I think it has mitigated their use in regional conflicts like, say, the Ethiopian invasion of Somalia.
Of course, proliferation isn't really a problem with CBU's, but if it convinces the nations who don't think they'll use them to disarm their stockpiles, what's the harm?
I'm not sure that's the case at all. The real problem children as far as landmines go - Cold War-era Eastern Bloc manufacturers and stockpiles, and the third-world armies and rebel movements that buy them - aren't impacted by the treaty at all. I suppose you could make an argument that it discourages weapons firms from making any
new ones, but again, it's not the new mines that are the problem.
The Ottawa treaty on landmines is merely a promise by signatory countries not to use mines, clear the ones they have, get rid of
government stockpiles, and offer assistance in mine clearance and medical care for victims. More to the point, most of the world's biggest suppliers and consumers (China, Russia, North and South Korea and a good chunk of the former Soviet republics) haven't signed, and a large proportion of FSU-made mines are in the hands of private dealers and firms anyway.
Assuming that the CBU ban uses essentially the same language as the landmine treaty (the same coalition is pushing both), neither problem will get solved, because they
don't take into account the root of the problems. It's a very naiive approach to the problem; as if "let's just all agree not to use these things, okay?" is a realistic way of regulating the use of very widely-proliferated conventional weapons.
And the very real harm of these kinds of make-believe treaties is they substitute the illusion of action for actual solutions.
As to your specific example - the Ethiopian invasion of Somalia - I'll tell you from experience, the Ethiopians are ruthless SOB's...if they wanted to use mines, you can bet your ass they would.