What better way to know your limitations than to seriously challenge yourself? I taught my friend to program in Haskell and while he struggled early on he found it incredibly rewarding. Since then he's found it extremely easy to learn other programming languages, despite their extensive differences from Haskell.

There's also a lot to be said for habit-forming. Imperative languages teach you a lot of habits and cause you to develop assumptions that might actually make it harder to learn a language as different as Haskell. This makes the concept of "taking steps" in your growth as a programmer a nonlinear one. Some languages might even hurt your ability in the long run.