I wore my tall black riding boots only once or twice last winter. They are close to being dead anyway (probably 5 years old) and are no longer super comfortable, which contributed to the non-wearing, but I did not really miss them. I'm not sure I'm going to replace them at all. They're a bit too classic for me. I personally can't imagine an outfit that would go with riding boots but not the Melissas, but I guess it depends on what type of look you are going for.
For other knee-high boots, I have: cognac Miz mooz laceups, flat black UWP-ish laceups (definitely NOT classic riding boots), tall gunmetal grey patent heeled boots with some straps and subtle studs, and really weird silver/black giraffe-ish print flat boots. I also have a bunch of mid-calf boots, including 3 black pairs, so it's not like I have no black boots. =P Anyway, these pretty much covered all my bases last winter.
ETA: Oops, I didn't actually answer your question. I would say no, they're not riding boots, but I don't think you need riding boots.