I'll offer up some thoughts (which may be contradictory) since this issue is something I have pondered after switching from battery to the MD and MPC for most of my beat work.
First, hardware synths take up space. Most people who are going to invest in hardware usually do it in the form of a synth -- maybe because synths can "do" more (as in make different sounds) or maybe because the idea of using a drum machine is antiquated in the face of software drum synths and samplers. The latter offer more control and detail over parameters and of course a larger screen to edit on. Which could lead to a debate about hardware vs. vstis, but I think that's another, albeit related, thread.
More importantly, however, I think most people think of drum machines in terms of, and most drum machines are designed in, the typical xoxo style (unless you're talking about a drum sound module, like a ROMpler, which would be controlled via a MIDI sequencer). As a result, it's very difficult, if not impossible to program really fine "drum lines" (which I put in quotations because triggering drums at 1/128 resolution usually leads to buzzing!
). Maybe I'm ignorant, but I can't think of a drum machine with x0x0 style programming that make such detail possible without messing with the tempo or time signature (which often is a global parameter. . . ?). Of course, you could always control the drum machine from a MIDI sequencer, but sometimes, as in the case of the MD, going this route would not permit you to take advantage of things like parameters locks.
So, perhaps its the way that drum machines are designed, and why the MD is such a special drum machine, because it allows they type of control over parameters that many people have been looking for in a hardware drum machine.
That said, the MD is still not 100% capable of the same things as a MIDI sequencer/software drum machine combo. For example, the only way I've been able to recreate the buzzes that I loved to make with Battery and a MIDI sequencer is by using the retrig parameters on the sampled sounds in the MD. Otherwise, in triggering the synthesized drums, short of using an external MIDI sequencer, I can't think of how to get the same buzzes or fine resolutions on just the MD. The resolution is just not fine enough -- but please, prove me wrong if it IS possible: retriggering an EFM drum sound at high resolution while taking advantage of parameter locks sounds lovely.
Anyway, just some thoughts for a sat. morning.
-M