Unless you truly are a one-man army - one must absolutely be paying attention to the tiobe index. Even if the worst possible language is on top, its the one language that can't "destroy your career". It's 10 times more valuable to be programming the same language in a company than be the one Lisp or D or Erlang guy that writes side programs that no one will help you maintain - and will definitely be re-written in Java when you leave.
