Poppagator

I know what you mean. The way I see it, you have 100% of effort so, of you paly and sing at the same time, you split your effort roughly 50/50 between the two. The playing and the singing might both be good but not necessarily great. If you do one or the other you can dedicate 100% of effort to it and have a better chance of being great.

This is true (for me) regardless of what kind of song you might be singing.