Single player content is important to me, but I also get a feeling every now and again to want some community time with people, so I'll go play an MMO shooter or RPG.
However, most of the games I own are single player games, and single player games tend to have way better stories than MMO games. Actually almost 100% of the time they do in my opinion, the only "story" I like in an MMO is World of Warcraft's story because there's just so much lore to be had.
I deeply enjoy SP games due to the story and the depth that most of them have to the game. It's deeply disappointing to see a SP game that's half-done and released with buggy features and barely a story, survival games non-withstanding for the most part. Mechanics are also a big thing too in SP games as I love to see what unique things developers can come up with for their SP games, and I love sharing strategies and stuff like that with friends and people on the internet.
MMO games are so, so different from SP games, as a lot of the time, most MMO games force people to play together when they really don't want to, or grind to outlevel the areas so they can solo them themselves... in which case they should be playing a SP game. However, a lot of times MMO games don't quite put together a system to fluidly put people together into groups to experience the storyline of the game and get loot. This makes for a frustrating experience for a lot of people who are anti-social. This is when Blizzard, World of Warcraft's developers, come and shine, as I'm pretty sure they developed the first auto-grouping system, their Dungeon-finder system. It's not without its faults, but it's extremely easy to run a dungeon without saying a word to anyone and get loot and go home. And this is only ONE facet of MMO games that I like. However, there are tons of other MMO games out there besides RPG games. There's survival games, shooters, just so many different things.
In the end, it's definitely a matter of personal preference for sure.