I added achievemnts in your topic so this could stay in the general gaming section. Consequently, I made a few small edits so that both system owners could talk about this. I removed the part about xbl having a trophy system. That's what the achievement thing is

They are called xbox achievements.
So, as for the topic, yes, it's absolutely a marketing move. As I said in my views on gaming blog:
Achievements/trophies are a great way for the video game companies to keep us buying and playing their games. They are a marketing tool, nothing more. When MS started the initiative, you think the idea was pitched because they were looking to extend replay value, or increase the bottom line?
This is how it probably went:
''We need to keep gamers playing more and more. How can we ensure that they stay hooked?''
''Well, sir, gamers love assessments of accomplishment (think points) and competition, right?''
''I'm listening.''
''Well, why don't we create a system that tracks accomplishments throughout all games, giving the user an overall look at how they are doing? We can assign this a point value, say, a gamerscore. So you accomplish something, say, earning x amount of kills, and we call that an achievement. These individual achievements across all games add up to a total comprehensive score across the gaming platform. Then we sit back and watch as the obsessiveness and competition leads to more and more sales.''
''Brilliant, Johnson, brilliant.' Here's a gazillion dollars and a harem of Brazilian prostitutes.''
''Ah, yea boiiiiiiii, I'm going all out tonight, yuppie style!''
See, they are banking on us acting like little mice, scrambling to acquire as many gamerscore points as possible, which is why they tack them on to expansions and why arcade game trials tell you when you've unlocked one, and prompt you to ''BUY NOW!!'' lest you ''lose'' the achievement.
All that being said, I freaking love achievements. They obviously know us pretty well

The only difference is, I don't let them affect my purchasing decisions. And I do not hunt them all down in games I don't like. I'm not going to labor through a shitty game for some meaningless points (although, to be fair, I used to. That's right, for a while there, I was sucked in as well. Luckily I at least have a rental plan, I wasn't buying games for points, but still). Once I have a game in my hand that I truly enjoy, then I'm going for those damn achievements, because at that point, they can be quite fun to attain.