The Souljourner said:
The "manufactured weapon" qualification is not actually a rules change. The definition of "melee weapon" on page 310 of the
PH already excludes natural weapons, such as an unarmed strike. This merely makes the rule less confusing.
The restriction to weapons with which the character is proficient is principally there to cover religiously-themed classes which specialize in their deity's favored weapon and otherwise have a very restricted list of proficiencies. By the RAW, that qualifies them for this feat. I attempt to make it more thematically appropriate for these characters.
Since the Fighter can take this feat AND greater weapon focus AND greater weapon specialization, it seems that the 12th level fighter comes out ahead. And with the great number of +14 BAB prereq. feats, the 20th level fighter definitely comes out ahead simply by virtue of having more feat slots to fill these goodies with.
Your analysis, I think, is incorrect. A rgr 6/ftr 6 may give up GWF, GWS and one other bonus feat, but gains two favored enemies, 36 skill points, four bonus feats, wild empathy, an animal companion and minor spellcasting. A multi-classed barbarian/fighter gains rage and better skills.
If any of those high-BAB prerequisite feats tempt the multi-classed fighter, he can put off taking fighter levels 5 and up until later in his career. That will give him as many high-level fighter feats as if he were a single-classed fighter, plus the powers of his other class. In many cases, it's not worth it for him to keep up his levels in both classes; a barbarian or ranger's high-level abilities are better if you stay single-classed, but a fighter's are only a little better.
In sum, this feat only makes multi-classed fighters even more powerful than they are now. It does nothing to make single-classed fighter a more viable choice for powergamers. There's no justification for it to be better than three good feats put together.