For example, many years ago, before I really got functional programming, I tried to learn ML and while I figured the syntax out very quickly, solving problems in a functional style was a lot more difficult and I found it hard to use the language. Learning to use Python or Ruby or Java or C#, albeit not idiomatically, is a hell of a lot easier.

Of course, knowing a language is a lot more than just learning enough to use it, though you certainly don't have to master a language to know it (and many people will never completely master their languages, despite knowing them quite well and using them effectively - for example, I'd wager few people have truly mastered C++).