I really like co-op in just about anything... but Mass Effect is ultimately about one person making these galaxy-altering decisions. The pacing of the overall game (between shops, conversation zones, explorations, and eventually combat) just doesn't work for it.
Co-op would be a bit dull for the second person, since I would certainly not want them doing much of anything aside from helping me shoot people. They might have dialog but I wouldn't want their inane mutterings to have any effect on the course of my story.
It's nothing like (for example) Resident Evil 5, which was almost entirely centered on the combat.