I definitely prefer keeping adventuring and non-adventuring competence separate, but I tossed out the option because some people seem to prefer them mixed.
While I understand that its more realistic and suits some people's tastes to have real mastery of a craft out of reach of adventurers, i would personally prefer to let them have it. For starters, not everyone is restricted to our puny three-score-and-ten human lifespan. Surely a dwarf can master a craft before picking up his axe and heading out. Heck, he's probably not allowed to have a drivers license or graduate high school until he does. But apart from that, many fantasy heroes are skilled in arts or crafts as well as war, and that's something I'd like to allow my players.
Really I'm not worried about people "abusing" non-adventuring stuff. I'll probably let my players have anything they can weave into a good character background. Craft, music, art, etc don't get nearly the spotlight time that combat, magic, or even social prowess does, so when it crops up, it should shine.
I got another notion while reading about the different tiers of armor (plate, warplate, godplate). Cap the highest level of craft ritual the player can learn at several points. If there is a skill, cap the amount your level adds to it (say at 5, 10, and 15); if there's no skill, have there be Apprentice, Journeyman, and Master Craft feats. Again, I'm working off the assumption that the DM simply grants these feats or skills to the character based on in-game events, rather than spending character resources on them. In order to bypass the cap or gain the next level of feat you have to craft something of extraordinary for that level. This could be great adventure fodder and make the crafting more a part of the overall campaign, rather than just something to do during downtime. You might have to go mine mithril in monster-infested caves, recover lost craft secrets from a dwarven ruin, find a fallen star for meteoric iron (a la OOTS), or get dragons blood to temper your sword. Once the work is finished, the character reaches the next tier and the campaign has gained an item with some built-in background.