The first entry comes from none other than IGN, praising Bioware for daringly introducing multiplayer to its spacefaring strictly single-player franchise Mass Effect. Based on the writer's points (though I previously agreed with him) he articulately lays out three concrete points presenting a valid argument.
In previous Mass Effect games, Shepard's companions acted more like bullet sponges than productive members of a team, leaving Shepard to do the hard work. Adding co-op forces the studio to carefully break down the core gameplay to work more efficiently with and without human-controlled players, thus improving the overall experience. Though, the question is whether co-op will ultimately improve the combat side of Mass Effect's gameplay.
Looking forward, Bioware would be crazy not to continue this franchise in some format. The writer extends his argument by saying adding multiplayer prolongs the overall experience of the universe while also giving players more value from their purchase. One fact he forgot to add, however, was a co-operative mode springboards Bioware to maintain this universe on a grander scale. The future of the franchise has been rumoured seemingly forever, and with Star Wars: The Old Republic launching soon, EA has built an MMORPG portfolio. A fully-realized universe shouldn't be wasted.
Lastly, Mass Effect is predominantly known as a single-player franchise. That's why the original announcement of multiplayer caused quite the stir. Bioware, hearing the calls, opted to make the mode entirely optional. This is great news considering there's a crowd out there who spent hundreds of hours specifically tailoring their own adventure, their individual version of Shepard. Though, not forgetting the series' roots, multiplayer is still story-focused.