I think that's rather unfair about the custom maps. There's been some great ones so far. Nexus Wars is quite fun, and at this hour thats the only one I can remember by name.
It's totally valid. Not so much because the map makers suck, but because Battle.net 2.0 is horrible.
If you like playing tower defenses or bad MOBAs, then you might enjoy custom games.
Edit: Current customs on page 1. Three MOBAs (Hero Attack/SotIS/Star Battle), three cat & mouses (Probes vs Zealots, MineralZ, Island Defense) two madnesses (Marine Arena, Bunker Wars), Phantom, Runling, 1v1Obs, Roleplay, and Special Forces. 1v1Obs is... well, 1v1ing with observers -- fun, but not far removed from ladder unforunately. Roleplay is... let me just suggest to you that you don't RP on Battle.net. Just don't. Okay? Okay. Special Forces is also a map that borders on mindnumbingly retarded.
At best, there's 8 types of games there. I'd personally discount 3 of those, leaving you with 5, but you could also argue that there's 8. Maybe you could go to page two and find some games there. Maybe 3? It's hard to get people to play on page 2. Now you're up to about 11. You could play tower defenses, but A: tower defenses are shit. B: tower defenses are largely identical. So you're up to 12. There's a few tug of war style maps that could get you up to 13.
I'll be generous and say there's about 20 different maps that you can play on Battle.net 2.0. And that's a pathetically small number, compared to BW/WC3.
If you like to 1v1 ladder (or 2v2/3v3s, those work too) then you can enjoy SC2. If you really like tower defenses... see a doctor, but maybe you could enjoy SC2 too. If you like MOBAs, go play DotA or LoL.