Finally, I agree aerobatic planes fly potentially anywhere vertically but they are not a statistically significant bit of the big picture. If they were, they would be introducing a new hazard, but aerobatic pilots generally choose places to do it where there isn't much traffic.
Well, a vertical block altitude of 2500" should generally be enough for a full sequence. The height of a loop is approximately 500" and a one-turn spin should cost less than 1000" so to say that we "fly anywhere vertically" is a bit of an exaggeration - unless you aerobat in a jet of some sort.
We do indeed tend to fly in places that see little through traffic though, try to get the best ATC service available (well, at least I do) and squawk either the assigned discrete code or 7004. There is an implicit expectation that ATC, where possible, will issue traffic warnings about aerobatics pilots when they know about them, regardless of "limited service due to controller workload" or the exact service that someone obtained, as keeping a lookout while doing an aerobatics sequence is a tad harder.
Fortunately we do change direction, altitude and speed a lot, making us a little more conspicuously than traffic that flies straight and level. Remember it's the traffic that doesn't move with respect to your windscreen that kills you.