I use beet pulp because it does help with a lot of things - weight gain first and foremost. While hay may be cheaper in the long run, getting quality
hay around here can be a problem. We had no issues with weight loss when we fed our own home grown hay - chiefly because we would set a round bale out and let the horses have all they wanted, whenever they wanted. We never let the run out.
Our hay fields would normally produce more than enough on our share to get us through the winter and a little more. It was good quality, grassy prairie hay. With the drought this summer, we got a whopping TWO BALES!
The hay we can get around here is not the best quality, although our feed dealer did finally find some that we liked. The beet pulp helps to make up for the poorer qualilty hay we had to feed - and allowed the supplement we used to stick to the feed instead of sifting down to the bottom of the feed tub. Fortunately, the horses like the supplement and would lick it up...but they also tend to tip their feed tubs over and eat the feed up from the ground (now why do that do that?).
Yes, beet pulp is more expensive than hay, but it makes me feel a little better knowing I'm doing all I can to help keep my bunch warm and hydrated.