I've been coding in Ruby/Rails for 3 years now, and been active in the community, and I haven't yet done that or encountered anyone who had done that. People are warned that monkey-patching is dangerous, they learn how to monkey-patch responsibly, and they act accordingly and responsibly - if something does go wrong with monkey-patching (which, as I've said, seems not to happen), the person who did the patching is aware of why things are going wrong, and doesn't blame it on the language - I can't remember reading a single query in, say, #rubyonrails, where the problem was due to monkey-patching.
But the problem is that all the hacks that don't poorly affect one module do poorly affect the composition of modules. Remember how you can't use Rails and some full-text indexing package because they both monkey-patch Array differently? That's a major problem.
Ruby programmers get themselves into plenty of trouble. Of course, so do Java programmers, or any other programmers. The problem is people, not programming language features. (Except manual memory management, as per C. That's a language problem.)
