I'm pretty new to bass playing, but usually play a 5er. I sometimes play with a dance band (glenn millar style etc) and find the key to be gflat, dflat or aflat. Trying to play these on a 4 string would be too much for my limited ability. At least on the 5 I can anchor myself somewhere and have the guts of two octaves to work with.
Sometimes the music throws up high f,g etc. and this is where I would actually like a 6 string (currently I just panic and leave it out) or maybe I'm lazy