According to PC Gamer, you'll get a pretty lousy ending if you don't play multiplayer in Mass effect 3. Apparently BioWare decided that the best way to "encourage" co-op play in the game was to tie it to the single-player ending. As the player engages in a war with the Reapers, he or she earns what the game calls "War Assets" and "Readiness."
You accumulate these War Assets from a culmination of sources, but the most prominent are the decisions you've made in the previous games and the ones you make in this one. Basically War Assets are the people and armies whose help you earn. PC Gamer has a bit more detail on the complexities of this system, but the end of it is that the way BioWare designed the game is questionable. Not everyone wants to play the four-player cooperative mode, and yet the game punishes you for not doing so by giving you an ending that is less than inspirational.
Some have suggested that BioWare did this because of piracy in that pirated versions of games don't tend to use online play at all because users are easily identified as having and using a pirated version of the game. Some pirated releases often strip out or disable multiplayer altogether.
Here's some of what PC Gamer's Tom Francis says:
"I did every proper quest I could find in Mass Effect 3, made sensible decisions that didn’t conflict with my choices in the previous games, and brought people together. But I still got a gallingly bleak ending.
That’s because I’d never played the multiplayer. It’s a co-op mode where you and up to three other players have to survive waves of AI enemies and complete objectives. If you succeed, you get an increase to your Readiness rating – a percentage by which your single player War Assets are multiplied by. These are specific to each sector of the galaxy, so if you have a lot of War Assets in the Terminus Systems, you’ll gain more by playing on a multiplayer map set in the Terminus Systems.
It’s all rather… dirty. Presumably they’re trying to encourage you to try the multiplayer because to do well in it, you have to buy or earn unlockable items, and you can get these for real money. But they’re doing it by hurting your single player game, slapping a good playthrough with a bad ending as a penalty for not playing co-op. Even if you like co-op, it’s not unreasonable to want to play through the single player first."
You can read the whole thing here.