I detune my low B to an A for the ease of octaves, so I use it all the way up to the 12th fret quite frequently.
We have two guitarists in my band, so I rarely (once or twice) use the C-string in that context unless it's some sort of area that cuts out and leaves room for a prominent bass part. I could get away with a 5-string in the band, but I love my tone, wide neck, and narrow spacing....why switch?
When I'm doing any sort of solo work, I play with chords and tap a lot, so my B sees very little use but my C is probably played most.
It's just a matter of situation I'm in as it dictates the need for string X.-