I am interested in bluegrass playing. I really, really enjoy listening to it, and would like to be able to make my own bluegrass. However, my main interest is and always will be metal. Will learning bluegrass, particularly if I learn it flatpicking, and learn to solo in bluegrass, help my metal playing?
It may not necessarily improve your metal playing (though it very well could), but in my opinion it can never hurt to have as many styles as possible under your belt, because the more you can apply to any specific genre, the more varied and interesting your music can be.