In the science of developing video games (and there is one, certainly), one of the keys to making an online game successful is to create motivation for players to come back to the game, day after day. In this case, it isn't a subscription based service; it's just a great game. However, Even the greatest of games get stale after a while, and they have to create SOMETHING new to make it interesting.
So, put yourself in Valve's shoes. How do you make the game more interesting?
As I see it, there are only a few ways. One of them is to create more maps, or new gamplay modes. As far as I'm concerned, the two go together under the category of changing the playing field. The other way is to tweak the classes, which changes game balance intrinsically. This also keeps players involved, by adding new dynamics that the players need to cope with to be successful. Think about it; have you been more likely to get killed by stickies recently because fewer demos are using them? They have, partially at least, regained an element of surprise that they originally had.
The other way you can change the game experience in this way is to add new classes. However, if the classes aren't constantly being changed, it doesn't matter how many you have; They will still become stale.
I can understand why some people gripe about the these changes; It makes the game more complicated than before. Back in the day, you never had to ask what a sticky launcher did; it was common knowledge. Now, people STILL ask about what the Scottish resistance does, despite it being out for a long time. If you watch the videos about TF2 design, published around when the game first came out, Valve strove to make the characters recognizable and distinct, so that you knew EXACTLY what was coming at you, and for the most part, the weapons had the same effect. There was no ambiguity as to what the class was holding, nor what effect the weapon had. It was just known, and automatic.
Now there's ambiguity. To the trained eye, there isn't. I know exactly on sight (and sometimes sound) what the different weapons are, and I know how to deal with them, largely because I play the game daily. with these kind of changes, this is precisely the market they're appealing to: the dedicated fanbase that plays often enough that the changes are significant in their own right. However, if someone, like the original poster, for instance, steps away from the game for a month, they can come back and be overwhelmed with the huge variety of stuff flying at them. And that, ladies and gents, is how these threads are born.
So what's the moral of the story? Well, as far as I'm concerned, if it wasn't for the class updates, TF2 would no longer be popular. Think about it: the medic and pyro updates were over a year ago. In fact, even longer than that. The updates provide huge incentive for players to learn to play a class, get rewarded with achievements, and to get some new swag to prove they're better than other players; it all works together to create a big rush of enthusiasm, and even sales for valve during update times. Do you think for 5 seconds that the game will not experience a massive population rise for the 2 weeks following the engie update?
These updates have, in my opinion, extended the life of the game considerably, and for that, I'm happy about them. They keep the experience, for me, varied and interesting. And, no matter what they do, thematically or otherwise, they will achieve their goal: players will have to adapt and change to fit the new weapons and learn to use them/counter them, and for that, they will create more interest.
So, that whole dialogue doesn't actually answer anything about whether updates are good or bad, but I hope (for those who were ardent enough to read it) that it helped to explain the impetus behind them. The question of good or bad is one that I refuse to answer because it's not only personal opinion, but it's also largely irrelevant. The average off-the-shelf game out there doesn't last for more than 50 hours of gameplay. I have more than 300 hours of TF2 since steam started keeping overall track, and more than that since before that; if they broke the game tomorrow, it's still the best 10 bucks I ever spent.
