Pretty much what the subject says. I recently started putzing around with a high-stealth knife build, and there's some natural synergy between using both thrown and hand-held knives. The issue is that Throwing Knives are considered grenade-like weapons, and as such they appear to have a fixed AP cost (15). The main appeal of knives is that they're fast weapons, especially when you're stacking Dexterity - but even at default values, the slowest knife (12 AP) costs less Action Points than it takes to throw a single throwing knife.
This isn't really game-breaking, of course, but it's something that really jumped out at me during my current test-run. Throwing knives certainly benefit from the scaling damage that increases with your Throwing skill, but their slow speed and limited utility makes them a bit sub-par as even secondary weapons. Ripper + Fatal Throw can certainly be a fun feat combo when it comes to finishing off badly wounded foes, but that soaks up two feat slots AND can't be acquired until Level 10. Pinning is certainly interesting, but is far from reliable (35% proc) and has a very short duration (1 turn). Split Spare is probably the best of the throwing feats, but it's also somewhat unpredictable (since it randomly selects your "secondary target"). Having all four feats can, of course, make throwing knives much more appealing - but again, that requires using up four of your precious feat slots, all to buff up a sub-par base weapon.
I think the ideal "fix" might be to simply have Throwing Knives follow the current Dex-based AP reduction scheme. Unless my math is off, that means that by Dex 10, it would cost about 11AP to throw a knife. Not a huge difference, but a notable one that makes sense from an in-game logic standpoint. It might also be ideal to have Tabi Boots reduce the AP cost for using throwing knives by 1AP (the same way it currently reduces the AP cost for melee attacks), since the link between ninjas and throwing knives is fairly obvious.
An alternate solution might be to introduce a feat that reduces the cost for using Throwing knives, though by itself it might not make for a very appealing feat (some secondary effect might be required to make it more appealing). It would also add to the already hefty investment required to currently make throwing knives more "usable", since it would be a fifth feat stacked atop the other four. A feat that reduces the AP cost to use all throwing utility items might be ideal, though, since then it would be appealing to all throwers (Nets, Grenades, Caltrops, Knives) rather than just knife throwers.
EDIT - An different "simpler" alternative might be to introduce a new "Knife Strap Belt", akin to the Bullet Strap Belt that already exists in-game. It could even have near-identical effects - one extra utility slot, +5 to Intimidation, and then an AP discount for using throwing knives (and their poisoned counterparts) in place of the reloading bonus.