That's substantially different from, "Hey! There's a strong, clear voice talking funny from that bush over there! Kind of a rhythmic chanting... oh, hm, how weird, a dragon from nowhere. Like... magic..." :/
The thing is, a given GM will rule what makes sense to them - as they should - but it can easily and effectively neuter schools.
Unlike, say, a frost-wizard in Irrisen (unless that location is entirely unknown), you're usually not signing up for a campaign where the very nature of the rules negates your character concept.
The problem is that it's so... hand-waivy.
That said, I do hand-waiving of my own, and generally enjoy it.
And the other side of the coin is that it is extremely easy for a GM to hand wave or create situations that minimize these issues away for NPCs and thus make illusions and Enchantments extremely powerful against the party.