I wouldn't want a 2wd compact tractor (especially one with a loader), but for larger field tractors there are a number of benefits to 2wd, if you are using them in the right application:
-lower purchase price
-tighter turning radius
-lower maintenance costs, especially front tires
-less complex/more reliable
-more options for row spacing
Between me and my family we make between 60-80ac of hay a year and the tractor that does all the cutting, most of the raking, and runs the square baler is a 2wd JD 2550. Its got good power, is heavy with a long wheelbase (better ride) and it turns sharp. We use a 4wd JD 5045e for tedding and sometimes raking, and a 4wd Kubota M9540 for running the round baler. I can't think of a time when we have EVER used 4wd on one of those tractors in the hay field. In fact, aside from loader work those two tractors only get put in 4wd maybe a handful of times in a year. For loading round bales with the loader in the hay field they are left in 2wd.
IMO there are only a few situations where 4wd is considered almost mandatory:
-heavy loader work or loader work on steep hills/muddy conditions
-work on very wet ground
For haying, bush hogging, chore work and general field work I gladly use 2wd tractors.