Look at the beginning of the section on P253 where it says "a mount and rider follow these rules while the rider is mounted." Then it goes on to say that mounts take a -2 attack penalty. So the rule is if a mount has a rider, then it takes a -2. Mounted Combat still works, it just removes that -2 (not sure if the other effects are still relevant). The feat also has plenty of RP implications of course, though that may not be much comfort to a lot of players. Truthfully it is a marginal feat at best unless you both have a mount with some very effective attacks (and generally ones it can take without using up the rider's standard action) AND the character is planning to spend significant amounts of time fighting from the saddle. I think it really should have some more significant bonus personally, something that really makes you extra good at fighting while mounted.