Buy them all.
Some spells will be locked out to you. For example - Pyro will never be available to a frost mage - you must pick it in the fire tree, then you can buy upgrades (Higher Ranks) of it. Since you don't put points into the fire tree - you can't buy upgrades for something you don't own...
Also - there are few things that better define a bad noob than not having certain skill sets. I ran into a mage who didn't own the Sheep spell because - "I don't use it, so why would I buy it?" I ran into a Paly who couldn't Rez people. (Bring back to life) The world is chocked full of hunters who don't have pets....
You might not use it, but a group might need it. Counter Spell (CS) is a good example - I never use it in Solo PVE and I'm not big into PVP. However - some group pulls an be difficult if you can't CC (Crowd Control) casters. Example - You and group are up against a 4 pull. Two casters and 2 melee. You will be asked to sheep one caster. The two melee will run at you and be pulled by the tank. The other caster --- (Guess what) will cast. You can CS (Silence effectively) the other caster and he'll run at you (Melee is his only option if he can't cast right?) - the tank will pick up the caster now and you've effectively CC'd one guy, and shut down another...
In short - buy em. They are cheap...