My opinion, a P style bass is the easiest way to fit a bass into the mix in a way that doesn't hinder other instruments. For most rock and pop styles, the bass is filling out the low end (up to around 250) and then some character and crunch around 1k. A P bass naturally has a nice low mid presence, and if you play it with a pick, provides a nice balanced tone. In the recording sessions I've done, I have always preferred the sound of my P (or PJ on neck) over most other basses I've tried.
I also think the massive drum kit rule applies. Theres less room to be flashy, more room to be solid, and less sonic range available. When I've worked with players who play 6ers or even some 5s, ive noticed that their playing is usually overlapping with either guitars, keys, or the deep end of the kick. Not saying you can't be awesome and solid on a 5 or 6 (see: Nolly), but my personal preference is that there isn't much of a need for more than 4.