Why do recipes call for the vanilla to be added last? In this case it is a banana cream pie, but also in candies. The simple argument I've heard is that the alcohol in it will boil off, but in my case my imitation vanilla doesn't even contain alcohol. (Uses propylene glycol, not really an alcohol, right?) 

But either way, why would we care about the alcohol boiling off. It's the vanilla flavor we want right? Does cooking it affect the flavor? (Cakes and such seems to counter that thought) are these recipes just misguided?
It's not the alcohol specifically - the reason alcohol is used to make extract is not because it contributes any flavor itself, but because it dissolves specific desirable substances from the vanilla pod. When the alcohol boils off (or whatever solvent is used in the artificial flavoring), you will also lose some proportion of the flavoring compounds dissolved in the extract.