Don't forget constant updates to tf2 bring a lot of exposure to the game. Generally, even large updates might add 3 weapons and a couple of maps, but it usually gets shown everywhere, and more exposure means more new players. On top of expanding the player base, by keeping the player base happy, they have a lot more guaranteed customers for any sequel down the line, or any Valve product in general, due to amount of support they can expect (the entitlement issue bit them on the ass with Left 4 dead 2 though, since Valve fans pretty much expected them to deliver on the first game like tf2, when really they should have been grateful for what they got. But that's another issue, and my parenthesis bound aside will get too bloated if I go on.)
It's an ingenious marketing technique, because I can see clearly what they are doing, yet I don't care. They're still giving us tons of free content, and exploiting it in the best way a company could.