Neverwinter Nights had something similar, where every 5 ranks in Spellcraft gave +1 on saves vs. spells and every 5 or 10 ranks in Tumble (I think it differed between NWN 1 and 2) gave +1 AC. It kinda made those skills mandatory to invest in. Maybe not Tumble for characters that dumped AC, but everyone uses saving throws as a defense. Granting direct combat bonuses like saving throw bonuses is really strong. You'll likely see Fighters using their 2+Int skill points on cross-class ranks of Knowledge (arcana) and Knowledge (local) just for an extra +1 or +2 bonus on saves against the most common foes fought. It's not like they're going to get much use from their skill points and measly skill list otherwise.
I have no objection to making skills more useful, but you need a light touch, especially if you're enhancing only some skills. The value of skill ranks is generally fairly low with respect to overall character power, so it's really easy to add small things that make a skill much stronger than the alternatives.
