Hay cubes can be given as treats, but they should not replace the unlimited hay in their diet. Long strand hay has important fiber which is important to keeping their teeth ground down and helping the movement of their GI tract (preventing stasis). As those fibers are digested, they actually pull against the sides of the intestines and cause greater peristalsis reactions than the shorter chopped hay which is in the cubes.
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry