Well the paladin in my group has made limited use of the mounted feat chain, but it has definitely been useful. Mounted Combat, specifically, is a very good feat for keeping your mount alive against typical attacks. The paladin manages to negate at least one attack on his mount every combat. There have been several combats where the paladin negated every single attack directed at his mount
A special mount's improved evasion is also very good against area effect spells, but it's easy to see where that solution does not work for regular mounts. Other than for historical "flavor" reasons, I'm not sure that there needs to be a solution for regular mounts. After all, there are a lot of magical or exotic mounts available that will be much stronger, or even immune, against area effects. Any class with some sort of companion should also have an advantage in this area.
The later feats in the chain are also very strong. The paladin in my group has Ride-by-Attack and Spirited Charge. In the last session, astride his griffon, he practically single-handedly defeated the ambush party of demons sent to destroy the PC's. Without the mounted paladin, the party would have been toast. Instead there wasn't a single casualty. The adventure before that he had another great opportunity to use his mount, and the adventure before that his griffon was very helpful, as well. It seems that the uses of mounted combat are generally limited to just a few times per adventure, but when the paladin is able to be mounted, he seriously kicks some tail.
So I guess my opinion is that Mounted Combat is definitely useful, just not always so. It seems very likely that, in an open area, a mounted combatant is more effective than an unmounted one. So in general if a fighter wants to be flexible and be the most effective he can in any situation, he should pick up Mounted Combat for those times when he can use it.