Maybe "allow" is too strong a word, but "encourage" probably wouldn't be right either.
Do players in your campaigns research and design actual new spells - something unique to your ruleset?
I remember back in my college years, I'd have players occasionally express a desire to tweak a spell already in one of the AD&D books with the desire to give it their own flavor, but nobody was designing anything truly new. I haven't come across even a desire to tweak a by-the-book spell in any of the campaigns I've run / played in in the last few years. Maybe it's a "lack of free time issue".
So, do you or your players research and design their own spells??