I am a fan of multiple species, but I don't just want to play a re-skinned human.
If I am playing as a Salarian, I would want to actually play as a Salarian, not just a human in a rubber suit. A Salarian PC would have an entirely different perspective to the universe than a Human, or Turian would. The lore mentions how their hyperactive metabolism, and rapid thought processes make the other species seem slow in comparison. There is no way a texture swap will capture that distinct flavor, and that is just one of the major Council species. Playing as separate races should be more involved than simply how you look, doubly so if you are talking about separate species that have an entirely different evolutionary development than your fantasy counterparts that are just humans, short humans with beards, humans with pointy ears, and big humans with horns.
Would playing as a different species for all of the SP campaign be something I would want? Yes, but I would also like a story that had depth to it, a narrative that actually reacts to my racial choice, and has a wide arrange of ways I can role-play the game; unlike DA:I. I would like to believe that BioWare could pull off multiple species successfully, and with meaningful impact, but their recent title doesn't indicate that such a thing is possible without sacrificing a lot of other elements. Really, the only way that multiple species could work satisfactory (IMO) would be if BioWare went back to a silent protagonist, but we all know the likelihood of that happening.
MP, like others have said, is the best way to get playable aliens that remain true to their depiction in the setting. Even though ME 3's MP had very little narrative behind it, the aliens still felt like aliens. When you played as a Krogan warrior of Tuchanka you knew that that is what he would act like. Being an Asari Justicar actually played differently than a human biotic, etc. Plus, MP is the best chance to see the more 'outlying' aliens as a playable character. Even if playable Asari and Turians where a thing in the main campaign, does anyone really thing we would have ever saw a Volus, or Vorcha as playable options?
MP might not be everyone's cup of tea, but it is the most likely platform upon which we can see the Mass Effect universe through an alien's eyes and have it actually be unique to that species instead of a re-skinned human. I am secretly (not really
) holding out for some playable Rachni and Elcor, something I know that will never, ever be an option in the SP, but does have at least a chance of making it into MP.