You'd be surprised how many people miss that point. There's a "let's just use Tor first" approach some people use that really undermine their anonymity. I.E. Using plugins like Flash while browsing and even JS. In the West, that's usually not a big deal, but elsewhere, people's freedom could be at stake if they don't take adequate precautions.
If you are using either TAILS or the Tor Browser button, both of those are intentionally impossible. Javascript can be enabled, but there's probably not as much issue with it as with Java or Flash.
