Exhodos: For people who want Mass Effect, it will be useless because the third will never be released outside Origin. And playing only 2 games of a trilogy is pointless.
IMO.
You can't watch A New Hope and The Empire Strikes Back without seeing The Return of the Jedi after that, whatever the quality of the film is.
Actually Star Wars works well enough as a standalone movie, which is pretty much what it was way back when. Keep in mind, it wasn't even an "Episode IV" back then.
The first Mass Effect may not be quite as standalone, but I think it's still perfectly worth playing on it's own. Actually, I'd probably recommend only playing the first one, before the series went full "shooter/dating sim OMG WTF Starchild!"