It seems that the MPC-2000 had a long stranglehold on drum programmers and sample editors, but with the rise of DAWs (most notably I think was FL Studio around version 4), are they now obsolete? What advantages does one have to using an MPC versus something like Kontakt or any built-in sampling device?
When you look at any piece of vintage gear, I think you are looking not only at a time capsule for music technology, but also at how that technology influenced the sound of music of that age.
For example, the Yamaha DX-7 had a profound influence on the sound of popular music in the 80's. It was a key component in the arsenal of a huge number of bands, albums and soundtracks of the day. I used to own a DX-7, and the sounds it makes are very distinctive; I can always recognize one when I hear it being used in a song.
Or, take the Yamaha CP-70 electric grand piano. You might well ask, what's so special about this piano? Well, it was the piano used in Peter Gabriel's song, "In Your Eyes." The sound is unmistakable; no other piano sounds like it, and there is no piece of gear or software plugin that will fully substitute for that sound.
So it is with the MPC-2000, or any other piece of vintage gear. There will always be those who choose that piece of gear because it has the sound they want, or the workflow that they need. And make no mistake, the workflow of the device (how the controls are arranged, the device's features, capabilities and operations) has a profound effect on the creative process.
I agree with Robert, first off hardware has its own character, and the MPC is well known for it. As far as I recall the groove templates in the MPC are particularly good. Another advantage is the restrictions placed on you by the hardware, you're forced to learn how to operate it, unlike software, after using Logic Studio for a number of years I'm pretty sure I could open almost any DAW and get a track up and running in an hour or two, something like the MPC is a totally different story. Restrictions force you to find alternative solutions you might not normally consider, and generally makes you more creative as far as I'm concerned. It's easier to develop your own style like this. It doesn't need to be vintage gear either. I'm a fan of both hardware and software, but certain things are better done on hardware than software IMO. While I do most of my composition on software, I do as much of my sound design as possible on hardware. I've never use a MPC to compose myself, but I've used a Yamaha RM1-x and it definitely made me think differently about how I composed.
The interface is very different: for some, working with buttons, a jog wheel and an LCD is preferable to a mouse and a monitor. I think the main advantage of MPC-like devices is that having a set of physical controls, that are mapped to a limited number of functions, is conducive to building up muscle memory--something which mouse-plus-monitor interaction is very badly suited to.
Speculation: So paradoxically, although some operations are more fiddly to achieve on an MPC than they might be in a DAW, it may be possible to execute them with less of a disturbance to your creative flow on an MPC. My hunch is that with MPC's, after you've used one for a while, muscle memory is doing a lot of the 'heavy lifting', while a mouse-plus-DAW combination can rely much more heavily on visual processing, which I can imagine might cost more 'brain cycles' (heavy use of keyboard shortcuts can mitigate this i expect).