My 1.5 acre plot in Transylvania, Romania, has been covered in alfalfa for 7 years. I purchased it this summer, and until that time, for 7 years, it had provided 3 crops of alfalfa hay per year.
Alfalfa is a legume, and as such, a nitrogen fixer, hence in principle it
should contribute to the quality of the soil. Also, it has extremely deep tap
roots (several metres long) that can mine water and nutrients from very deep down - as a result, even in the hottest & driest periods, the alfalfa is constantly lush and green. That means, for instance, that you will have a source of mulch (or fodder for animals, or biomass, or green material for composting, or... etc.) throughout the season, all you need to do is mow it down. That's what I have been doing.
On the down side, a field of alfalfa is heaven for
voles, as it provides perennail food both above and below ground, and long-term
shelter from birds of prey. This autumn the plot was riddled with holes and tunnels that the voles had dug... Also, alfalfa is indeed said to be allelopathic, and as it grows it tends to outcompete other herbaceous plants
My plan is to gradually phase it out. I've disturbed the land in several areas - by making swales and other
earthworks, and ploughing strips for tree planting - so alfalfa will stop growing there. But elsewhere I'll keep mowing it to use as mulch under the
trees and on the vegetable garden beds.