Portability between implementations is a bit of a red herring (as you seem to have discovered); most libraries you'd actually want to use run on most implementations you'd actually want to use. Ruby and Python don't give you this either, but they do give you a standard implementation. I suspect a lot of Ruby code you might want to run doesn't work with JRuby or IronRuby.

Portability between operating systems is a more serious problem. I think ECL is the only free implementation that does this well, and most libraries don't seem to be tested on it.