From what I understand, a decent thief needs very high skill (200+ %) to have much success at all. Why should they be relying on a secondary skill just to level up?
Most if not all other types of characters either rely on a secondary skill or don't level till level 21.
I can't speak of others, but I level my crafter alts till they have the 3rd level of the desired profession.
With IN 10, that's around level 7 which is easy to get to doing mostly quests.
No need to level them further.
My doc alt is more of a crafter build than a fighter, so the same applies for it.
Although out of personal preference I do level my doc to level 21 by using it's SG skill.
By taking perks you can add 70% to your steal skill.
Also the time out for stealing is 30 seconds when you fail or when going to another target.
Other time-outs are longer.
It would be interesting to see the xp per minute for things like crafting ammo, armor, doc, FA, steal, lockpick compared.
Assuming stealing from npc with an average of 3-4 items (including stacked items) it would be 30-40 per minute. That's assuming you're only stealing, not placing items on npc and taking stacked items as 1.