For me, a complete BCG wasn't that much more than a bolt. May as well get the whole thing so you aren't wanting for gas key and screws, firing pin, firing pin retaining pin, etc. When it's a $30 or so price difference, just get it all. Easier to swap out too.
The downside is that if you have spares of the hard to get parts, those tend to work their way into becoming complete rifles. Amazing how these things grow.