I used to play a 4 string, and i'd play basslines, and play the routes along with riffs and etc, "holding down the low end", but when it got to a chilled out, relaxed bit of the song, I wanted to play something nice and soft that wasn't making your ears flap, and I couldn't go high enough for this on my 4 string. I got the 6 string because I didn't realise (I was young) that I could have a 5 string with a high C. Now I have the 6ers though, I use the low b to add to my low end playing, adding in fifths I couldn't reach before, or playing fat low roots too low for the E string.