I know it's there for a purpose, but in practice I often find so much (length) of it that getting it back into the box can be a challenge, and using 1.5" of it (a 2x2 is only truely 1.5x1.5, in practice it will be even less as the wire can move diagonally) to move the box forward isn't going to materially affect the lenghth of wire behind each switch or outlet.

One office at work lost power because whoever wired a junction box in the ceiling in the 80's didn't leave enough slack and as the wire shrunk it pulled right out. I had to pay real money to an electrician to find the problem. Point being you can't take anything for granted until you see it.

The wire had "shrunk"? Does it get that cold in winter? .... Or did it get wet?

No, it didn't get wet, but there is shrinkage, (maybe the insulation?),and that's why they're supposed to leave extra, according to my friend the electrician.

At the time, the best price for 1/4" sheets here was about $8, it was $5 in Buffalo and the dollar was at par. The proportionate saving was the same on the mould resistant and fireproof sheets but the numbers were bigger. We made two trips, one to Buffalo, one to Detroit, and brought back everything for finishing a floor; drywall, glop, bathtub, taps, lights, etc. The saving was worthwhile and we got to visit exciting places.