You're looking at it completely wrong. It stands to reason that if you only advance non-combat skills, you won't get better at combat. It doesn't stand to reason that if you advance non-combat skills, you'll actually get worse. Technically, you're not getting worse as it's the enemies that are scaling up, but it's the end result. A level 10 character with 100 speechcraft, lockpicking, or whatever, has actually ruined the game balance for themselves.
At first I thought along your lines. After all, why should non-combat skills advance your level at all. Then I started seeing some of the stuff those skills could get you and realize that they do offer a tangible increase in your combat effectiveness. However, there does seem to be a "soft" spot where the benefits don't compensate for the increase in diffiuclty. It's unfortunate but, again, this is why the slider exists.
I would also like to note that in another 10 levels if that character focuses on combat then he;s right back in the saddle with no real drawbacks. The game expects that you'll dabble in crafting and doesn't make rediculous skill demands for combat.

