I love them. They're awesome. I hate the name for them though. Underleveled. It makes it sound like they shouldn't be that level. Why?
So what if YOUR Zigzagoon doesn't evolve at level 7. I don't see why a wild one couldn't. Biker with a level 20 Muk? So what. His Grimer wanted to evolve before yours even thought of it.
There's a reason the anime ignores level. It's a ridiculous mechanism. In an organic world, level cannot exist. Its too organised. The games have to be organised, calculated and run smoothly because they're games. If Pokémon Diamond said your Chimchar should evolve at level sixteen and Pokémon Platinum said it should evolve at level twenty, that'd cause some problems and some people would complain that their friend's Pokémon got stronger faster just because of the game they picked and it'd all be hectic and annoying. Its data, it has to be organised. However, they can throw in the odd "underlevelled" Pokémon for variety's sake. Its only standardised