I was referring to abuse, not robustness. Like any other form of syntax or semantics alteration (operator overloading comes to mind), there are good reasons to have the opportunity (operator overloading comes to mind), but making use of it just because you can will easily create a maintenance nightmare (operator overloading comes to mind). And operator overloading is a relatively mild form of this. Perl 6 will allow really drastic interventions.
I am excited about the possibilities, but I hope people use the opportunity carefully. It'll most likely turn out to be one of those things you should not do unless you know why you not.
Makeshifts last the longest.