In my gaming group, Mounted Combat has long been regarded as useless. Our GMs don't often execute attacks directly on mounts, so we've never actually seen the feat used, despite a large number of characters taking it to gain access to Ride by Attack and Spirited Charge.
On a larger level, this is an endemic problem with feat pre-requisites; a poorly designed feat serves as little more than a stepping stone to the functional feats that follow it in the chain. As GM, I often waive prerequisites in order to circumvent this. What I prefer to do, however, is to make the initial feats more appealing.
Here is my house-ruled Mounted Combat feat, with a followup feat we designed to make the mechanic more appealing, even without Spirited Charge or Ride-by Attack:
Old SRD Feat:
Mounted Combat
Prerequisite: Ride 1 rank.
Benefit: Once per round when your mount is hit in combat, you may attempt a Ride check (as a reaction) to negate the hit. The hit is negated if your Ride check result is greater than the opponent’s attack roll. (Essentially, the Ride check result becomes the mount’s Armor Class if it’s higher than the mount’s regular AC.)
Special: A fighter may select Mounted Combat as one of his fighter bonus feats.
House Ruled Feat (changes in italics)
Mounted Combat
Prerequisite: Ride 1 rank.
Benefit: Once per round when you or your mount are hit in combat, you may attempt a Ride check (as a reaction) to negate the hit. The hit is negated if your Ride check result is greater than the opponent’s attack roll. (Essentially, the Ride check result becomes the mount’s Armor Class if it’s higher than the mount’s regular AC.)
Special: A fighter may select Mounted Combat as one of his fighter bonus feats.
Improved Mounted Combat
Prerequisite: Mounted Combat, Ride 10 ranks.
Benefit: As Mounted Combat, only you may attempt a ride check to negate a number of attacks equal to your Dexterity Bonus.
Special: A fighter may select Improved Mounted Combat as one of his fighter bonus feats.
I'd love some feedback on my Mounted Combat fix, as well as any horror stories you have about stepping-stone feats. Tell us what you do in your own groups to fix this!
I've certainly seen Mounted Combat used in the Core Coliseum (PvP combat on the WotC message boards). Just because your DMs aren't into mount-killing doesn't mean that no one's DM is.
At any rate, I think your replacement feat could be too powerful; it's generally much easier to get an additional +3 to a skill check than an additional +3 to AC (especially touch AC). YMMV.