Environmental concerns? Cost of oil? Having to wash the fuselage more often because of the oil streaks?
In addition to those, keeping track of oil consumption is one of the things you do to keep track of the overall engine health. After a break-in period the oil consumption of an engine should stabilize at some low number. If that number starts to rise all of a sudden it's time for some TLC.
If you regularly overfill the engine you can't keep track of what was consumed and what was blown overboard.