Yes, "tankie" is a derogatory term, specifically against Marxist-Leninists. If you weren't referring to Marxist-Leninists, then it doesn't make much sense to use the term. Why do I call myself a Marxist-Leninist (I assume that's what you were asking)? Because that's what I am. I believe in the political theories of Marx, Engels, and Lenin (and Stalin), making me a Marxist-Leninist.