It's not a gimmick, it's quite simple actually. A Glidecam is simply a fulcrum system, where both ends have to be equal weight. You buy the product with the most suitable payload so that you can comfortably accomodate your camera's weight range.
If you're using a bare-bones t2i/t3i/60d or a GH2, the 1000 is ideal. If you're doing strictly 7D/5D work, the 2000 is ideal. If you're using video cameras, the 4000 is ideal.
Having said that, you can use DSLRs with the 4000 no problem; the weight of a DSLR just happens to fall under the lower end of the payload spectrum, meaning you will have plenty of room to add weight or use heavier camera systems, but not much room to go any lighter. The same applies to using a 7D + 11-16 on a 1000. You may be able to balance it, but you will be maxing out its payload, which inhibits the possibilities for rigging variations.
Some people who shoot strictly DSLRs buy the 4000 because they plan to upgrade to video cameras later on, thus future-proofing their purchase. Once I upgrade to an actual video camera, I'm also going to upgrade to a HD4000.
I will also add that I feel the HD series is a phenomenal improvement over the Pro series. The Pro series is a pain in the ass to balance and overall feels kind of cheap. The HD series has horizontal  micro-adjustment knobs and gives you the ability to pop your camera on/off without having to rebalance each time.