In essence, I think you're correct: embracing multiple languages is something of a mess. It's the kind of mistake that programmer's are prone to, they're inclined to think that supporting multiple languages is just a technical problem, when really it's also a social problem, and having to deal with code bases that are mixtures of perl5 and perl6 will tend to produce subdivisions in the perl community (which already, to some extent, has to deal with a certain amount of sub-specialization in C, SQL, HTML/XML, and so on).

All of that said, though, it's worth remembering that perl6 really is intended to seem like "perl", so the cost of context-switching between the two is likely to be lower than it would be even for languages like Ruby or Php (which have clear perl roots). And stasis isn't really an option for programmers: even if we all decided that perl6 was a bust, and we all stuck with perl5 for the foreseeable future, there would still be a constant stream of new modules to become familiar with and so on.

I would venture to guess that switching between perl5 and perl6 code will be easier to deal with than, say, switching between Class::MethodMaker and Class::InsideOut code.