I just started doing it as well, as suggested by my vet clinic. The biggest reason is that I just hate messing with the stupid tubes. I have to deworm one draft, one draft cross, and several others, and it's just too complex messing around with partial doses in those stupid tubes.
Plus some of the horses are very adept at spitting out the pastes. The gel in the cattle dewormer is pretty much impossible to spit out.
The fact that it's also way, way cheaper doesn't hurt my feelings! And yeah, there's the waste issue, too.
However, I will use the other dewormers again, just to rotate chemicals and prevent the parasites from building resistance.