You're not blowing your level-boosts just to be good at a skill. It's assumed that skills you put effort into mastering - skills you expect to regularly hit hard DCs for - will be ones linked to stats that are already important for your class. It's the same reason smart characters like knowledge skills while dexterous ones like sneaky skills. An item or other investment is just a way to further cement the player's desire to make mastery of said skill part of their character concept.

And I guess we're looking at the situation differently. If most of the battlefield, say, is covered in ice, the DM is trying something really low if the PCs have to roll against a high DC. If only one strategic corner is, to make the rogue's player happy he invested in Acrobatics, that's something different. There's a difference between feats of derring-do you attempt to gain an advantage, vs. ones you're forced into. The former should have the DC set high enough to discourage every tom, **** and harry. The latter should work hard to avoid autofails.