In many areas alfalfa is unavailable or cost prohibitive - the pelleted/cubed form can be found pretty much anywhere (even if the local feed store doesn't carry it, they can order it in). Many people choose to use pellets/cubes as a supplement to grass hay and find the processed form to be more convenient.
For me, it's a combination. Alfalfa is available here, but is somewhat pricey. I am lucky enough to have a source for a nice blend that is mostly grass but has a good bit of alfalfa in it. Rather than having two separate bales of hay to feed out of every feeding, I opt to supplement the grass/mix hay with alfalfa pellets which are cheaper than purchasing the bales outright.
I stick with the actual alfalfa. Here it is cheaper and easier to get enough for long periods. All my horses are very good at self regulating as they have free access to big bales (mixed with grass in summer and pure in winter). They are fat but healthy and we have never had an issue with either colic or founder since we stopped feeding sweet feed.