I obviously didn't explain every point of the system as well as I should have

There are a few points I'll have to clear up to really answer your concern.
First, the idea behind having two sets of circles is to allow the player to focus either on the element of the spells or the spells' actual effect. So, if you want all the destruction spells you would simply spend all your points (or almost all of them) raising Destruction all the way to tier 9. Then you would be able to research all the destruction spells (not including those that have a cross-over requirement like lightning spells). However, you wouldn't be able to research any protective spells so there is a definite trade off there. From a magical point of view you would become a glass cannon if you see what I mean.
Second, your sorcerer doesn't “level up” like characters in an RPG. You're not going to get X experience and gain a level. Now, your units and heroes do work like that, but they don't effect the spells you can research. Gaining new circles or reaching higher tiers in circles you already have is going to depend on luck (a little) and hard work (a great deal). You will be able to find items in game that will either open a new circle (if you don't already have it) or raise your tier in a circle (if you do already know it). You will also be able to spend TONS of research to open new circles or gain higher tiers.
If you were to do what you suggested in your post and open every circle you would only have enough points to open them at tier 1. That would have a rather interesting result. You would be able to do the following:
Research every tier 1 spell.
Research every tier 1 cross-circle spell.
Research every tier 1 combo-spell.
Research every tier 2 earth destruction spell. (tier 1 in earth + tier 1 in destruction = tier 2 E&D).
Research every tier 2 fire destruction spell.
Research all tier 2 elemental (including life and death) destruction spells, but no tier 2 destruction only.
Research all the tier 2 elemental summoning spells (but no tier 2 summoning only.)
Etc.
You see the strength (and weakness) of this strategy? You would have an extremely wide range of spells you could research, but only at very low tiers. Now, this will be a playable strategy because the mana cost will be so low and you will be able to cast the right spell for every situation. However, you have to keep in mind that you can only cast one spell a round. So, although you will be able to hit your opponent's units with just the right spell, you're not going to be able to blast the battle board with Meteor Storm like you could if you had tier 9 in destruction. It's going to be a trade off.
I think the system works because it supports both extreme (tier 1 in all circles or tier 9 in a single circle) and “middle of the road” play. Say you took 5 tiers of summoning and 4 tiers of fire. You would have a wide range of low level summoning and fire spells and all the way up to tier 9 fire summoning spells. (4 fire + 5 summoning = 9 FS).
I hope that explains the system a little better. Let me know if you have more questions.