There is nothing wrong with "mandatory" talents per se. Generally the community considers them mandatory because they are very good talents. We don't want a bunch of trees with a bunch of mediocre talents. We love the depth that comes from players endlessly debating which spec to use or comparing talents to each other.

Where it crosses the line in when it starts to restrict player options too much. It's okay if almost all warriors take Cruelty and slightly less okay if almost all warlocks take Ruin. If there was a 31-point talent that all specs took, then it starts to hurt the other two trees in that class. Cookie-cutter specs are okay when they are something used by players who are new to a class and don't want to spend the energy just yet learning every nuance of the class. Ideally, there are a few points that come down to player preference. As an example, before Lich King the three tanking classes had very choices in their talents -- they needed to spend almost all their points just on mitigation.

The extreme alternative is something where every talent tree is equally viable. That might sound good in theory, but it makes the discussions a lot less interesting. "What's the best talent build for a Balance druid who wants to do 5-mans and Arenas?" "It doesn't matter, dude. Just take whatever and you'll do fine."

So we usually land somewhere in the middle -- talents that you pretty much are always going to have, mixed in with some that come down to player preference.