We are both not very big fans of picking a weapon at first level and sticking with it throughout your career. We have yet to see a PC use a lance on horseback. I'd like to see a more dramatic Conanesque use of weapons and it strikes me as unrealistic when a party sells a +3 Bastard Sword because they already have +2 specialized, Flaming Longsword wielding fighter. One player in our group has even declared he will never take weapon focus because he considers it a waste of a feat.
Who said you have to stick with a weapon at first level or even after taking Weapon Focus? The feat just means you are better at using it than other weapons. If you don't use it, you don't get the benefit, but you don't have to use it all the time. You don't use other feats all the time either. They all have their time and place.
If you haven't had a PC use a lance, have you given them the opportunity to do so? Do the PCs ever play knights or anyone in a position to use lances? If they regularly play wizards, rogues, healers or adventure underground, underwater or in other restrictive spaces, lance use is practically impossible. When was the last time they were in town when a jousting tournament was going on? Offer a prize they might be interested in and the chance to win it if they participate. That's a good enough way to get them interested, if not having them all do it and end up jousting each other! Show them how impressive lances can be by witnessing a squad of knights routing a group of orcs or ogres (and reading off damage scores done to the orcs or ogres).
As for selling weapons being unrealistic, how is it so? If the fighter in question uses sword and shield, he's not going to want to take the time and effort to learn a bastard sword one handed (Exotic Weapon proficiency feat) when he could just sell it and get his own weapon improved, get a more useful magic item for himself or the group or something. And if I were a fighter specialized in longsword and had a +2 flaming longsword, well, that's already a +3 weapon, so what's the big deal about getting rid of a weapon he doesn't want or knows how to use? Selling that +3 bastard sword could give him the cash to get his +2 enhancement bonus on his longsword bumped up to +3. If no one can use or want the item, why not sell it? Conan used whatever weapon he could get his hands on, and that was fine for him, but if your PCs aren't like that, so what?
As for the player vowing never to take Weapon Focus, that doesn't mean there's anything wrong with Weapon Focus because of that. There are too many feats out there and characters get too few feats considering what's available, so if he doesn't want it or think its not worth it for his character, that's his issue with the feat, not anyone elses.
If you want to make the feat more useful and want people to start taking it, change the way its used in your games. Make it a prerequisite for a prestige class that someone wants to take. If it makes sense in your game, allow the bonus to apply to weapons of a similar nature or group of weapons like others have suggested already. Or have the feat do more. Maybe it could give a +2 bonus to initiative with the weapon. Maybe you could increase the threat range by 1 point with that weapon. Or have the bonus apply to damage as well as attack rolls--and if you do that, have the bonus stack with Weapon Specialization.
The balancing factor of broad weapon types -- or even of single weapon types -- is DR. ...Your proposed change reduces this balancing factor...
How does applying Weapon Focus bonus to a group unbalance DR? DR is already unbalanced in 3.5 because of all the different materials required to hit different monsters. Applying the bonus to a group won't impact that at all. There are maybe a handful of monsters that take more or less damage from a particular type of weapon (skeleton being the only one that immediately comes to mind). If you have Weapon Focus Mace or Weapon Focus Longsword and apply it to each respective group, its not going to affect your combat with the skeleton at all. It will still either take the reduced or full damage regardless of the bonus to hit. In your example of werewolves and zombies you're not considering that anything that works on a werewolf will work on a zombie equally well, no choice has to be made there. And if you're high enough level to fight groups of golems, you're going to be able to dish out enough damage to hurt those devils too even if you're not using silver.
If all you want is weapon variety, start by introducing new or different weapons into the game. Maybe the group is starting off at 1st level and they are assigned to city watch. Well, if the watchmen all use pikes or halberds, they get issued theirs and maybe encounter a few bandits trying to get into the city. Voila! Different weapons introduced and everyone gets the chance to try them out. Maybe an elite group of knights use morningstars or flails instead of longswords. There are all kinds of ways to bring new weapons into the game that don't need or require the reworking of a feat. Weapon Focus is a good enough feat already without having to be tweaked, unless that's just what you want to do. Consider that it lets a 1st level Cleric or Rogue (or other class with +0BAB) fight with that weapon with the same measure of skill as a Fighter of equal level (without the feat). That makes it a worthwhile feat for any of those classes, and even for warriors it gives them that much more of an edge in battle, whether its with just one weapon or a group.