Personally, I find that people spend countless hours playing games like CoD. And it seems that a substantial number of players purchase these add-on map packs the instant they become available as they tire of playing what came with the game. I think from an entertainment cost, dollar per hour, people get their monies worth out of the game and the map pack purchase.
Of course, I would like to get the add-on content for free and just have a better $50-$60 game in the end without spending any more money. On the flip side, would I be willing to pay around $15 every 6-9 months for some new content. Perhaps.... seems like a lot of people are. It's nice that you don't have to buy the map packs and you can keep right on playing.
At the end of the day, I just want to get my monies worth out of any purchase. If I am willing to pay $10 for a movie that lasts 90 minutes, that comes out to 11 cents per minute of viewing. So, if you take a game like CoD which lists for $60 (xbox version), you have to play for 545 minutes to get to 11 cents per minute. Serious CoD players will log 545 minutes in 2-4 days.