Continuous Content
Usually, my co-op buddies and I get through content fairly quickly. If it’s story-based content (like a story campaign), we often don’t enjoy going back and redoing it over and over. MMOs hold an appeal for us because not only does an MMO ship with a pretty huge amount of content to get you from level 1 to max level, but the normal model for an MMO includes free content patches (and later, paid expansions). Content patches add things such as new and exciting features (things that often didn’t make the cut for release), and often focus on adding more content the end-game players. This means that when we finish all the end-game content we can manage, we often still have new content to look forward to in the future. This ties into my last reason:
Economy
A lot of people think of MMOs as being more expensive than other games. There’s the box price (which includes a free month) and then a fee each month after that to continue having access to the game. For some players this kind of model ends up being very expensive for what they’re getting out of the game (such as players who only have time or interest in playing a handful of hours a month) For others, though, who log on several times a week for decent-sized playing chunks, it can end up being pretty economical. Some months I’ve gone without buying any other games because I’ve been so occupied with my MMO, while other months (when I wasn’t playing an MMO, I found myself buying more games. So for me, personally, it often helps me cut down on my gaming-related expenditures. Let’s not forget that there are plenty of free MMOs out there, too - and free is certainly economical. Note, however that most of these have their own separate cash shops, so be wary that in some games players actually end up spending much more money than a normal monthly fee. There’s certainly no harm in trying them out, though, and seeing if they strike your fancy.
Those are a few of the reasons why I MMO co-op. What are some of yours?