You will probably get a variety of answers on this one.
Personally, I don't feed it full time & wouldn't, even though mine have free choice mineral. I use it as a treat a couple times a week.
I would say most people don't, but there are some that do. It is more expensive around here, and technically, while they are gestating, that would be a lot of unneeded calcium. I have read that too much calcium before they kid and start milking can confuss their bodies and cause milk fever once they kid. I am not an expert on this subject.
Some breeds probably do better with it than others, some breeds would probably get fat on it, I know Nigerian Dwarfs have a much lower energy requirment than some of the small thinner dairy breeds.
This is the first year in 15 years of raising goats that we have fed any alfalfa in any form to our meat goats, during milking/nursing and we were happy with the results. weaning an average of 50lb twins at 8 weeks. the does sure lilke it, I did notice that once you feed alfalfa on a regular basis they aren't as quick to munch on that grass hay. They will eat every speck of alfalfa before they would ever consider touching our good quality 2nd grass hay, that they were perfectly fine with before they starting getting alfalfa.
I feed alfalfa free choice to my goats. It is the cheapest hay here most of the time. If for some reason I can't get alfalfa, I use bermuda grass, also free choice, but the goats don't like it and they waste a lot.
I feed free choice high quality alfalfa to my does as per my breeder's recommendation. No issues at all as far as being overweight. They also get Purina goat chow which I believe has alfalfa pellets. If they had the choice they'd strip the bark off a pine tree and eat dry leaves from last fall.
ETA: They are lamanchas, not sure if breed makes a difference.
Short link from *i think* msu states that feeding either legume (alfalfa) or grass hay is equally acceptable but states differing mineral requirements based on hay type. http://extension.missouri.edu/publications/DisplayPub.aspx?P=G3990
I was feeding alfalfa free choice but they were getting fat so I switched to hay that's alfalfa/grass mix. They are doing fine either way. I'll probably switch back to higher percentage alfalfa hay once they're in milk again--makes for richer tasting milk I found.
My goats don't really seem to like Alfalfa that much. They get free choice Hay, and I lure them into the barn and onto the stanchion with grass based and alfalfa pellets.