Wow, some strong opinions in this thread.
Aesthetics, you can't really judge people on. If someone prefers the looks of a 4 string P, then that is their perogative. Of course there are many extended range basses which can sound much like a P, though I can see the concern in some contexts. If I wanted to start a Neil Young tribute band, I might wonder whether the 9-string bassist at the audition will be able to contain himself in the heat of the moment, or indeed whether he'd be comfortable making his hi-tech handcrafted bass-machine sound as raw as the material required. Likewise, you might have a preconception or two to get around if, as mentioned before, a 7-string medieval weapon turned up to your western swing audtion.
Both my basses are 5's, though very much in the lineage of Fender designs. They would rarely look wildy out of place, and at and audition after a few bars I hope I'd be able to prove that I can get an appropriate sound and feel for a song, to the point that people won't worry about how I get those sounds. I only go below a low E when I feel it's warranted, and when it happens, it generally gets a positve response. Most musicains enjoy a tasefully used low note, and most punters seem impressed to see a 5'er, as if it is some kind of achievement!
I think that most working musicians understand that the musician is more important than the instrument, and if you can serve the music well, then you're in.
Almost paradoxically it tends to be the hobbyists who feel the strongest (either way) about this kind of thing, though why the 'pro-bashing' I'll never understand.