The only benefit mentioned is that beet pulp promoters claim it is good for hind gut health, so that has me very curious. Anyone care to elaborate on that? Does the quick digestion mean better hind gut health? I always thought slow digestion was better.
No, quick digestion isn't usually better and I don't think the promoters mean beet pulp is better than hay, but rather they often are comparing beet pulp to grain products people feed. If you were to feed a horse beet pulp, which is digested as a roughage/fiber to add weight to a horse versus a grain product, which is at least partially digested in the foregut, the beet pulp is going to give the horse better hind gut health.
"Easily digestible:" take a horse that has difficulty keeping weight on, and someone who is feeding lower quality hay with a lot of indigestible lignin, the beet pulp is going to add more energy to the horse's diet. Also it is more easily digested if a horse has poor dentition and does not need to process the soaked beet pulp as much with the teeth.
Of course long stemmed hay is better when possible, because of other effects such as moving sand through the gut and preventing colic, etc. That indigestible fiber does have a purpose, unless the ratio is so high the horse has difficulty getting calories.
Beet pulp is very low in phosphorus, so especially if you feed alfalfa which is high in calcium, you can throw the calcium/phosphorus ratio off if you feed too much.
In my mind beet pulp and hay cubes or pellets are quite comparable.
If I had a horse with bad teeth, I'd feed beet pulp rather than hay cubes. If I wanted to add weight to a thin horse that could still chew, I'd feed hay cubes or pellets instead. That would be if the horse was having difficulty eating enough good regular hay to gain weight, because he could eat the pellets or cubes more efficiently.
Currently I give my horses a little beet pulp daily, because it is the easiest way to mix their vitamins, some of which are powdered, and make sure they get them all.