Now on the one hand that is good news for World of Tanks: There are far too many games which aren't inherently fun enough to keep people playing, and the various rewards and advancements are all that keeps you at them. If I play light tanks over the others it means that at least for me in World of Tanks I keep being motivated by playing for fun, and not just by playing for advancement. Which is good, because all these games of advancement suffer from the fact that advancement always ends at some point, some "level cap" or "maximum tier".
On the other hand I wonder if playing for fun isn't a sign that the pull of advancement is weakening. It used to be that only role-playing games had levels and stats. But these days pretty much every game is a game of advancement. I'd even say that some developers are making games now which are ONLY about advancement, failing to put any fun gameplay in. It has been shown that this constant stream of rewards and advancement works because it makes the brain release dopamine, a neurotransmitter that provides feelings of enjoyment and reinforcement to motivate a person proactively to perform certain activities. So some scientists say they have evidence that by playing a lot of computer games with a constant stream of rewards, our dopamine receptors are being weakened, leading to both video game rewards and natural rewards losing their power of motivation.
The downside of this is that we risk not just burning out of one game, but burning out of all games of advancement. Less scientifically expressed it appears logical that your millionth virtual pixel reward excites you a lot less than your first one. It would also mean that the efforts towards gamification are doomed to failure, because by the time we have turned the real world into a big video game, we have kind of lost interest in these games and their rewards. The upside of the development is that the games which will survive this trend are those which are inherently fun to play, with or without advancement. For me that would be World of Tanks right now. I'm still waiting for a MMORPG which would be fun without the advancement.