Well, speaking in a general sense and not a mage sense, going purely into one tree can be just as advantageous as going deep into one tree as going into a 2/3s, 1/3 hybrid, as is going into a 50/50 hybrid, depending on which class, which trees, and what situation.
I know that it's not crazy for 0/0/61 frost mages and 0/0/61 resto shaman. The reason is that these trees are saturated with both great talents and good talents. On the other hand, people tend to only put 41-44ish points maximum in an elemental shaman tree because there's some talents in the resto tree that helps them, and there's few good talents left in the elemental tree after those that they picked. You'll often find 41/0/20 elemental shaman, and you'll find 16/45/0 and 0/45/16 shaman than full 0/61/0 enhancement shaman (are there even that many talents in the enh tree?)
You can find almost pure shadow priests, and hybrid disc/holy priests. It's not the case in the World of Warcraft that 55-61 points in only one tree is always a noobish thing to do. It's always a matter of what's where.