James McMurray said:The problem is that PCs and Cohorts are not the only ones capable of gaining levels. All characters and most monsters can gain levels.
Sloppy phrasing on my part, I should have said "advancement as a result of play".
XP is not only earnable via adventuring. You gain it for any encounter. Under the right circumstances, getting a bowl of milk for your cat could earn you a few XP (say for instance the milk was still in the cow, and you don't have any legs or wheelchairs available). Sounds like at least a CR2 encounter to me as far as challenge level goes.
This is basically what I was saying. If you expand the definition of "challenge" to include anything the DM deems convenient, that's not very different to saying you should hand out however much XP (and levels) is required to fit the needs of the campaign. You've just provided a handwave to let it fit within the framework of the rules.