Sorry, but in my experience this is not the case. In my experience in Dungeons and Dragons, in all its incarnations, horsemanship has been very important in the campaigns I have played in and the campaigns that I have ran. If you think that DnD is only about a ten by ten foot stone room with a kobold guarding a chest then you have been playing the wrong campaigns and under a very lame and unimaginative DM.
That isn't it. There are plenty of feats that a player can choose from to enhance and make good use of a horse and wholly implementable.
Mounted Combat
Mounted Archery
Ride by Attack
Spirited Charge
Trample
Also there are a number of manuevers on can do which can be made into feats, given that is how NWN works. Also when facing a greater number of enemies a horse can attack as well as its rider if the Riding skill is high enough.
In the campaigns that I have been involved with a horse is no less important than monstrous mounts. In fact we never dealt with Monstrous mounts at all. Our various horses, from our light warhorse to my own heavy war horse was very much important to the group's survivablity.
You need to read up on horsemanship in DnD. It does more than that by a long freaking shot.