The modern 5-string banjo with more than 20 frets is a 19th Century invention, a modification of the fretless African instruments brought to the Americas by black slaves. The short 5th string was common in those banjos. The high-pitched note of that string had two important effects: first, it added a drone note to whatever else was played; second, it meant that the melody was played on the lower-pitched strings. These effects influence all 5-string banjo playing.
The capo-ble banjo is quite at home with all the 5-string styles: bluegrass, clawhammer, melodic, etc. It has two obvious advantages over the usual banjo. (It has more advantages, and also some disadvantages. Watch this space.) First advantage: you can capo to any fret, thus changing the key of the instrument. Second advantage: you can fret the 5th string, even close to the nut.
Capo-ble Banjo 