Responding to how many jobs get ruined by a small dip, my friend had a whole bathroom floor fail. The tiles cracked along a hidden low spot that a longer level would have caught. He spent way more on repairs than he would have on a better tool.
Honestly though, how many jobs actually get ruined by a small dip? Most installs seem to turn out fine without that extra step. It feels like avoiding a problem that was pretty unlikely to happen.