Squish also helps coping with the numbers. The upper limit Blizz is trying to avoid is the 32 bits unsigned integer upper limit (4 billion and something). Next xpac, if nothing is done, will have to go above that limit and, on 32 bits systems, calculations like divisions on 64 bits, will make systems take performance hits.
Plus number inflation is not good in general. Why waste time and power handling those?