I think so far I have played only one fighter that ever bothered to take Weapon Focus at all. It doesn't matter if they were single- or multiclassed. Whenever I wasn't certain what kind of magic weapon I might be able to afford or find, I preferred other feats.
But then, I also never played a two-weapon fighter, and I think these are the ones that need these feats the most.
Still, Melee Weapon Mastery is clearly better than Weapon Focus and Weapon Specialisation or their Greater/Improved variants. A +2 to attack should not be underestimated, either. That means approximately 10 % more damage per round, which is a considerable benefit.
I think expanding the weapon focus/specialisation feat to all weapons of the same damage type makes a lot of sense to balance it with these feats, and it still gives those fighters that specialised themselves a considerable advantage - they can now switch weapons a lot easier than previously..