My philosophy on whether to introduce a new rune or not for a certain setting is based on whether that setting considers the concept to be a single concept or a combination.
For example, in my planned Atomic Kingdoms setting based on an advancement of Privateer Press's Iron Kingdoms, gun mages use a Gun rune, while arcane meckaniks use a combination of Death and Machine for their weaponry.
(When I posted about this setting, someone pointed out that Machine could be considered as a combination of Metal and Motion.)
So, to answer the question of whether there should be an Illusion rune:
In Atomic Kingdoms I would say probably not, since there do not seem to be a type of mage that specializes in illusions, and thus spellcasters probably would combine Befuddlement and Light if they wanted to cast such a spell. If I were converting my Audor campaign, however, I would have an Illusion rune, because this setting has a school of spellcasters who specialize in illusions.
An analogy would be human languages: something that is a single word in one language might be an entire phrase in another. Schadenfreude is one example: this one word in German requires a whole phrase in English (unless the English-speaker just takes the German word, as English-speakers are prone to do; that would be the equivalent of importing an Illusion rune from some other culture that has one.)
Hmmm, a Shadenfreude rune sounds intriguing...
