C&C draws from all editions, so you would think that its fans would also do the same. If the idea is to promote C&C, then don't push off potential fans by putting down any one edition. Most fans I've spoken to (but not all) go back to prior editions of D&D when adapting to C&C. Nothing wrong with that.
Few adapt from d20, which I think is a shame. How many times do we see threads on these boards where someone is looking for that rules-lite d20 system? There's a decent demand for this sort of subgenre, as seen with Microlite20, Quick20, etc.
To me, C&C should market itself not only as an old-school game that works great with AD&D and basic D&D, it should also market itself as a rules-lite d20-based system. Don't put down the d20 system. Just say, "Hey, for anyone looking for a rules-lite variant, give this a try."
Personally, I think it's great that C&C works with older editions. I know I'm having a blast pulling out old tomes. However, I feel that C&C should be approached as a system that is so well-rounded that it could work with any edition of D&D.
Imagine a game where you are going through a classic module and you're playing a truenamer. Picture a game where you're using the 2e Arms & Equipment Guide side-by-side with the Spell Compendium and Magic Item Compendium. Or a game where you're playing in Eberron or Ptolus and using materials from the AD&D Unearthed Arcana.
This is my vision for C&C. A basic rules set that allows you to play quickly, to customize your game, and to use materials from all editions of D&D.