Actually you would get out of options if you spam anything at all except basic skills. That means you can’t cast Fireball freely since it would hinder your ability to react quickly with a defensive skill. Also, there are ways to increase mana regen or reduce spell cost, which could easily cause imbalance issues for that sort of mobility/control/invulnerability skills.
Some moderate defensive skills that consume main resource may work on classes that replenish it actively, especially Barbarian I think, but not when you just have to wait. It would be even worse than cooldowns.
For a ranged class like the Sorceress, the best alternative imo would be a secondary resource like the DH’s Discipline, separated from main resource used for attack skills and impossible to regenerate easily.
There’s of course the possibility of cumulating charges with longer cooldowns, so you can cast several times in a row but have to be parsimonious.