I oftentimes avoid working with people who describe themselves as a [Rails|JavaScript|Python|etc] developer, for a few reasons.

First, when you're a [language] programmer, every problem is solved with [language]. If there was one language that was perfect for everything, we'd just name it "programming" and be done with this whole debate.

Secondly, and more importantly, I've never worked on a website or app where only one language or technology was used -- and that's completely ignoring the importance of skills like design, usability, product management, databases, etc.

One-language-programmers tend to have a "that's not my job" approach to anything that isn't their language, meaning code from them often isn't wired up or well designed or whatever.

(Naturally, there are some amazing one-language-programmers out there, however my experience tends to be the opposite.)