I think "playing offence" can be effective in cases where the thing you're fighting for is already winning.

By "playing offence" the OP essentially means taking control of the language of the discussion so that you get to specify the definitions and the terms. If you can do that, you have almost won. The usual problem is that the opposition is not entirely stupid and will not allow you to do that without a major fight.

Should one try to define the terms of the debate? Of course, a political argument is often about nothing else.