Well his spotting is useful, but then again marking isn't something I've ever had an issue with, with D-Horse though, he can distract guards just as DD can, you can even hold up vehicles in mid transit by galloping up to the driver and pointing your gun at them.
With stuff like Tanks, you can slip down one side of D-Horse as you ride up next to one, roll off and then Fulton the tank before it can even respond to your presence.
By being very minimal in functionality and very quick in movement, he becomes an incredibly valuable ally who won't get in your way, but I guess it just depends on how you play.
I've just found that he seems very underrated and my Missions have had the most efficiency and success when just having D-Horse.