The accepted mode of getting otherwise unobtainable information is to go visit the cranky old hermit living in the mountains. It's just the sensible thing to do. So, naturally, everyone takes their monthly excursion to the hermit's hovel to consult him on everything, from lock-jaw to lovesickness, necromancers to nasal viruses.
Now, if everyone's always visiting the poor old hermit, there's going to be an enormous queue... "Wellcome to the Hermitt's Hovele, Please Take Ye a Number and Have Ye a Seate" reads the sign outside the packed dwelling.
Imagine the poor hermit, having retreated into the mountains to escape this precise situation...
If a creature is transformed into another creature (say, a human into a dog), then that creature will become more and more accustomed to the new form until a point is reached where reversing the spell is impossible. A second transformation is possible, but the creature will have to go through the whole process again.