He's not saying "I want to play a Thief! Only those six skills! How dare they force me to use a Combat skill!"
He's saying "If I have the Thief stone active, my Combat skills will increase too slowly! When I'm level 25, and the monsters have scaled to level 25, my -30% Combat skills will be too weak! I'll get slaughtered in combat / gimped! The game is forcing me to keep running back to switch to the Combat guardian stone so that I can keep my Combat skills up to par!"
So yeah.... most of the stuff you guys are arguing about has nothing to do with the original thread topic.
...for the record, I think he's over-worrying, and that there won't be any problem like that.
The OP's worries have no real backing simply because Skyrim will not use the level scaling that Oblivion uses. Not only that, the OP ignores that the Thief archetype/Stealth Specialization skills do have combat options, regardless of how high or low your weapon skills are. To top it all off, the OP is ignoring the detail that +30% is absolutely insignificant in the grand scheme of things. Especially when compared to the +122% that you could get in Oblivion with the Major Skill + Specialization combo.
As long as there's no skills in Skyrim that take 70+ hours of swimming (I.e. Athletics), the percentage increase will be a subtle but nice bonus. Nothing too drastic or game changing.