Any reason not to upgrade to next tier?

Hojieko, on 25 September 2010 - 12:00 AM, said: People who stay in light tanks all their life are idiots. Light tanks are stepping stones to bigger better tanks. The only good thing they could do is find enemy and die maybe kill SPGs.
People who stay in medium line of tanks are wannabe go everywhere type, except problem is you can't go anywhere when you're moving below 40 kmph and when there's 2 heavy tanks waiting for you infront.
People who go into heavies want to provide the best support for their team-mates and wants to dominate the leader boards.