The non-warband composition quite often brought armies with a single cheap hero and loads of troops. While the warband rules can be annoying at times (you've got 50 points left when filling out all warbands..) they can also allow/force you to field different models than you usually would (those 50 points could bring a captain without warriors, but you're more likely to upgrade a captain to some great but expensive named hero, get a banner, take some more elite troops, etc). For example, I'd never have taken Glorfindel in 500pts with the old rules, but he fits right in with the warband system.
Another advantage is that your army is set up slightly more neatly, and any dicerolls with reinforcements are a lot faster than when you rolled for every single individual model.
The rules aren't perfect, but work just fine. Would personally prefer to have it accompanied by some restrictions as far as alliances go, but at least the opposing army now very clearly indicates what kind of player you're up against.