You pose an interesting question. I think gear is accumulated for three different reasons.
1. Collections. As someone on here once said, no one asks a stamp collector why he needs more stamps.
2. The belief that we are one piece of gear away from taking perfect pictures or one piece of gear away from eliminating gaps in our equipment portfolio.
3. Sometimes different cameras are required for different purposes. Sometimes this is true, but sometimes there can be a lot of overlap.
I've been thinking about this because I've been contemplating selling my Canonet. It provides no more convience than my Bessa (I still have to carry it around my neck), but the potential quality isn't as high. To expand on this:
If convience is evaluated on size, meter, AE availability, batteries req'd, film loading easy, lens availability, and focus style.... and quality is evaluated on build quality, longetivity, viewfinder, available lens speed, and available lens quality, we may see results like this (size of circle is price):