template mixins vs. string mixins

[Trass3r]

In the past template mixins were a neat special usecase of templates. Now  
with the "mixin template()" syntax they've become a separate thing because  
you can add special code for handling them, e.g. allowing them to add  
constructors to classes.

The question is: what is their right to exist? Is there anything you can  
do with them you can't with string mixins (or vice versa)?