On a banjo, the 5th string AND the first string are both often heard as "drone" strings. The first lick of Foggy Mt. Breakdown needs both the open 5th and 1st to sound like the lick we all love. If you substituted the low guitar E for the high 5th banjo G, that just isn't a good trade, if you're trying to sound like Scruggs style banjo picking.

I'm not saying a banjo tuned in 6-string guitar tuning doesn't or can't sound "good", it just won't sound much like the banjo sound people love. Perhaps someone has created or will create music that needs a 6-string guitar-tuned banjo to be played "right". Maybe you will be one of those people!

So whether you keep the 6-string or not, if you want to sound like a banjo, with the famous Earl Scruggs roll, you'll need the five-string version with the high string, or thumb string, as John Hartford liked to call it.