[Cohen]

Globals are a long investment, like Clamming.
Only they're easily "killable".
Some globals are very powerful, like Arcane Nexus, and Forge of the Ancients. Some others are aimed only, like Purgatory.
Others aren't so good because they could be useful to you, but damage many other players, so they're so rarely casted. (like Second Sun)

About dispelling, the two most important thing are:
Dispel is anonimous.
Dispel could be cast in "communion".

Personally I try to shoot down Forge of the Ancients and Arcane Nexus if I see them casted, even if I'm at peace with that nation.
However I've to use my gems, and it could happen that your resources are needed for something else (like you're at war with Ermor and you need harbingers and such stuff).

If you're at peace, I try always to get 1 gem producing spell that isn't Spell Focus (it gives a too low gem income, but I can think about too if nothing else is avaiable).