[color:"orange"]Hold the button to charge the spell and release to cast it.[/color]

In that case, a simple click should cast the spell a full strength (so you would only have to bother with holding the mouse button down when you wanted to cast something at partial strength).
I'd rather be able to select different versions of the spell for different magnitudes, have some kind of strength slider or use modifier keys when the spell is cast.

[color:"orange"]Proficiency gives players the opportunity to specialise in everything, which is an oxymoron.[/color]

I wasn't thinking of proficiency for determining the strength or level of a skill, just in determining the mage' ability to cast it (and perhaps an influence on accuracy). I mentioned being able to attempt to cast a spell at a higher power than normal, but there could be other factors which could interfere with magic. Noise, light or physical distractions (direct attacks, poisonous mists, extreme weather) could interrupt spell casting or increase spell preparation time. The more familiar a mage was with a spell, the easier it would be to cast out of habit or by reflex (lower preparation time), so the less distractions would hurt.

Perhaps spells learned from a book would have a higher initial chance of misfiring. If you learn from a teacher they can instruct you more closely and make sure you are competent. Perhaps some teachers could offer additional training (at a high cost) in their field of expertise to help you become more proficient. For example, a healer might be able to train you from a novice to a beginner, but you would have to find a master healer (after using healing spells a fair amount) to train further.

For offensive combat spells the proficiency could be based on the experience gained from defeated opponents multiplied by the ration of the damage done by the spell to their total number of hit points. Select trainers could help boost your proficiency, but in general you would have to earn it legitimately. You couldn't become more proficient in a powerful spell by casting it on weak opponents, since they would be giving little or no experience points.

The skill system I'm thinking of would still be based on skill points, so you could still dump a bunch of points into a new skill and have something effective, and wouldn't be penalized if you wanted to switch styles late in the game, from not having used a particular type of spell much before. Adding proficiency (as a lesser effect) allows the skill system to be more realistic and opens up some interesting possibilities, but it is also more complex than either a straight skill point or proficiency system would be.