The mpc has a great sampler, it should do you just fine for sampling loops and drumhits. In fact, looping on something like an s950 is probably harder than on the 2000xl.
Racksamplers do have a few things going for them though:
1. Sound, most people use the s950 because they love the sound of it.
2. Multisampling. Although you can do this on an mpc, it's much easier on something like an s2000.
3. Filters. The mpc has one my racksampler about 30 nuff said.
4. More room for sounds. Although older samplers only have a few mb of ram, you can use it more efficient than with an mpc. My 2000xl can only use 4 pgm.'s at a time. With a racksampler you can load 16 kits of sounds, one on each midichannel, and use them all at the same time.
And because you can do so many things to the samples, you might not need 50 snares to find one that sounds good.
Bottom line: you don't really need racksamplers, but they do come in handy every now and then.
They're also really cheap right now. The best kept secret to me is still my Yamaha Tx16w with Typhoon OS. Sounds just like an s950, but has twice the memory at half the price. I saw one one ebay for $50 today, but i already have two of them.
When buying an older sampler, try to get one with all the options and max. memory installed already. Most older parts are hard to find and very expensive.
