I found it annoying that I could (and often needed to) so easily in NWN2. I think one of the fundamental problems with the D&D rules is that once you cast a spell, it is gone (of course they somewhat fixed this with the fact that you can somehow memorize multiple copies of the same spell).

I much preferred a system like Ultima where you could cast anything you wanted, so long as you had the regents and the mana.