Well, I do hate generalizing because it's bad most of the time (generalizing about generalizing ftw).
The answer about gemming is entirely dependent on your gear combination. This is why I like things like Toskk's Bear Time to Live Calculator.
http://druid.wikispaces.com/ToskksBearTTLCalculator
Inputting your raid stats while buffed, talent points, etc, clicking on Gems from the drag down list, and clicking Generate. It will tell you which stat would best fit a socket of a particular color. Incidentally, at least with the gear I have in my secondary set (tanking - and not what you see in the armory atm), Enduring Eye of Zul IS better than a Nightmare Tear (even if marginally). Why? Well, that's simple and two fold: I'm already well over the DR for Dodge. So, defense adds one avoidance factor outside of dodge: +miss. It also adds more stamina than a nightmare tear.
So, if I am attempting to gain a socket bonus (with a yellow gem) like agility... then, actually, the Enduring Eye of Zul DOES make sense. I gain not only avoidance that isn't on DR, I gain more stamina -- and I gain slightly more crit and a very tiny amount more armor. The tradeoff is a very small loss in HP. The variance, however, is not huge enough either way to say it's a "bad" idea. In fact, markedly so, the two are so close in TTL that it's really a question of preference at that point OR the fight mechanics of specific fights. One or the other will be favorable in cases where the other is not.
In blue sockets, yes, stamina makes the perfect choice. In red, yep, agi/stam. In yellow, "that depends". And that depends on gear, stats, talents, preference, and specific fight mechanics. I don't think most feral druids these days are gemming entirely AGI gems in red sockets outside of DPS. If they are, I'd say L2P.
Agility provides more than just dodge. It provides crit. More crit = more savage defense. This translates to the non-druid person as 'block'. The more we 'block' damage that we can't dodge/avoid, the better. If you ignore your own innate defenses (be it dodge, missed, or via mitigation - Savage Defense/Armor), you cripple yourself as a tank.
Just because we
can gem entirely as stam doesn't mean we should or that it's a good idea.
Edit for some phrasing clarity.