Weapon focus and weapon specialization are both very useful feats. On the other hand, there are a lot of good feats available to characters so they're not "every fighter must have these or he's useless" abilities like specialization was in 2e.
I have found that weapon focus is very useful in several situations:
1. You're a bad guy. Really. As a DM, I've found that a lot of the feats that are useful to players aren't useful to bad guys.
-Power Attack? Sometimes.
-Cleave? Not too useful for the bad guys really. If he's taking down a lot of PCs he'll probably kill the party with it. If he's a scrub, he won't be taking down PCs so it won't do him any good.
-Sunder? This is a great bad guy feat. Before 3rd level, it's a lot easier for a player to get healed than for the player to fix his masterwork weapon. After that, it's impossible to restore the enchantment on a broken weapon. This will scare players.
-Improved Sunder? Even better.
-Weapon focus and weapon specialization fit into the sunder category of incredible feats for bad guys. They let your bad guys hit and do damage without damaging suspension of disbelief ("So, every one of these mercenaries has an 18 strength?") or forcing you to use higher level bad guys ("So, all of these bad guys are 6th level fighters?")
2. If you're built around the use of one weapon.
If your character is an archer, for example, you're probably focussed on using the composite longbow. You have point blank shot, precise shot, and rapid shot. Weapon Focus and Specialization will add a whole lot to such a character.
Similarly, if you spend a feat to learn to use a bastard sword or spiked chain, weapon focus and specialization make a lot of sense.
3. If you use weapon finesse.
You obviously don't have too high of a strength or you wouldn't use weapon finesse. Weapon specialization is one of the ways to deal significant amounts of damage.
For other characters, it may not be a good idea.
I have a fighter/wizard character who uses a glaive and then switches to a light flail and shield for close up work. He has a very limited number of feats and switches weapons quite often depending on the situation (glaive, light flail, composite longbow, and wand of magic missiles). He could take weapon focus in one of his weapons (probably glaive), but he'd probably only get to use it 1/3 of the time he's in combat.
Right now, he has:
Power Attack
Cleave
Dodge
Combat Reflexes
Quickdraw
(next level he'll pick up expertise)
All of these feats contribute to his effectiveness in just about every fight.
(On the other hand, a friend of mine plays a fighter who uses a longsword. Since he uses that just about every fight, weapon focus and specialization are excellent feats for him).