DMH said:
The book itself uses them to specialize soldiers- like heavy and light horsemen trading shield and shield and heavy armor, respectively, for weapon focus or skill focus (ride) and both.
I could be persuaded to allow Skill focus (ride), but no spell focus, no way.
As for myself, I can see arcane casters who trade in their weapon selections for a metagmagic feat
Again, no way in all the hells. They give up almost nothing (they might use a crossbow very early in their career, but that passes quite quickly) for a big boost
run, great fortitude, iron will, lightning reflexes or some feat that ties into the character's background.
Again, run might be OK, the others are a big boost and worth way more than weapons, especially to a spellcaster
Clerics could gain improved turning while giving up heavy armor.
You mean extra turning? That might be OK unless they use divine feats, then they again get a big kick for something they wouldn't have used, anyway (not that clerics are in any need for goodies, anyway)
That does bring up an important point- I would not allow a character to give up light armor when medium and/or heavy is possessed and martial weapons have to be given up before simple.
Even then, this means often that they really give up nothing and get a big boost out of it. If anything, I might allow Skill Focus (and there, I might restrict the skills to be boosted), Run, Endurance, Toughness (the normal one, not Improved Toughness or a houseruled Toughness that gives +1/lv), Eschew Materials (but then again, spontaneous casters get that for free IMC), Track, Brew Potion, Scribe Scroll. Nothing else.