Btw D-Walker sucks, something tells me it's unfinished and rushed due to Konami pressure. Otherwise I can't explain the uselessness.
It's about as fast as D-Horse, is more maneuverable, is much more resistant to fall damage, has interchangeable manipulators, and also has interchangeable heads (one of which allows you to detect enemies hundreds of meters away). You can't "side hide" on D-Walker, though, and that's a big deal. It exchanges inconspicuousness for, well, everything else.
I think the real problem with D-Walker for most people is that they wish to use the tools it offers as though they aren't aboard a large and somewhat noisy (not hugely noisy, but also not silent; slower speeds are quieter, though) robot, but in fact they are. It also is generally a better fit for non-ghost approaches, and is fun to play around with.
I agree though, if you're ghostmaxxing it tends to be more of a liability than an asset, and encourages risky behavior, unlike the horse. I had some success with it, but you must know the mission well.