How much does a level 60 MP heal pet heal again?
MP cost is one of two factors (the other being play style which I won't get into on this post, but will if the discussion steers that way) in choosing the right tome to use for you and Geb Book costs too much (even with my legendary pet that heals 28 MP). So, if you're in a situation that requires constant healing, Geb Book will make you wait for it to regen before you can use it again: time you don't have.
If you're a careful enough guy who doesn't get hit frequently, it would make sense to use Geb Book since you won't be tanking and rushing.
If you find yourself getting into situations where you can benefit from extra def, Prot would be the way to go.
I ran through glands with a T6 tome one day just for fun. I was expecting to nexus very early into the glands, but I ended up on the other side just fine. I went back and forth three times just to make sure it wasn't a fluke. My MP healed enough for me to be able to heal myself whenever I needed to, and sometimes I had enough Mana to heal twice.
That is why I ask about MP heal; it's essential. It can allow constant healing, making Prot feel not so useful anymore.