With pro detailing/correction you're not paying for the materials and a bloke standing there with a machine, you're paying for the skill, knowledge and technique. A lot of people think that correction is a piece of cake and that anyone can do it (no-one on this thread so far, I hasten to add!), but the level of skill actually required is very high, not to mention knowing when enough is enough so you don't wreck the finish!
That said, a DA machine will probably get you close to where you want to be, but ultimately nothing beats a rotary. And therein lies the danger of removing too much paint...
Blackhole is good stuff, but ultimately it's just hiding things. That's not to say I don't use it, it certainly has its place, however it also has limitations.