Don't Grant Free Feats
Instead of granting the feat for free, simply note in your house rules that spells that require inexpensive material components no longer require them. Then also note one or more of the following:
- All creatures are proficient with unarmed strikes and don't provoke attacks of opportunity for making unarmed strikes.
- A creatues can take a penalty up to its base attack bonus to all attacks in a round to gain the penalty's absolute value as a dodge bonus to Armor Class; no creature can take more than a -5 penalty via this ability.
- A creatures can take a penalty up to its base attack bonus to all attacks in a round to gain the penalty's absolute value as a bonus to melee weapon damage with one-handed weapons or double this absolute value as bonus to melee weapon damage with one-handed weapons used two-handed or two-handed weapons; no creature can take more than a -5 penalty via this ability.
- A creature can take a -5 penalty to its Str ability check when initiating a bull rush, a -5 penalty to the opposed attack roll when making a disarm or sunder attempt, or a -5 penalty to the touch attack roll to start a grapple or make a trip attempt to negate the foe's attack of opportunity normally granted by these actions.
The reason to do this instead is that by granting the feat Eschew Materials (PH 94) you grant the actual, for-reals feat, and anything that checks for that feat finds it. The same and more goes for giving creatures the aforementioned feats Improved Unarmed Strike (PH 96) or whatever.
The power that comes from granting the spare feat isn't the issue (because, seriously, who cares?). What is an issue is, as the DM, you've just made all published materials incompatible with your game. Any time a fighter shows up, you've to remember he has an extra feat... that he might've used to get into a class other than fighter. Further, giving out the actual feat means every creature has an additional resource they can attempt to exploit, purge, or swap.
In other words, instead of replacing a flavor ability with a mechanical one that makes characters harder to build, makes your world incompatible with all published materials, and forces you as the DM to remember to give even the disposable ogre Ftr1 an extra feat, you've said, "Hey, if you need to take the feat Eschew Materials for a prestige class or whatever, you still have to take it, but you don't need it unless you want that class."
That way, players and monsters get more options at the table rather than more options when building characters.