I think many style-of-play issues have to do with the notion that "the party" is a singular entity always made up of pretty much the same characters of about level X (continually rising over the course of the campaign).
You've hit a good point here.
Ariosto said:
That seems to be a very common approach today, and an eminently practical response to some needs. It's problematic (if understandable) when folks take it to be ubiquitous or somehow "the right way to play."
It was not the assumed scheme in early D&D, nor has it in fact been universally adopted. Some of us still run more or less what what was meant by a "campaign" in the original context.
::raises hand:: Guilty as charged, y'r honour.
Multi-interacting-party campaign, levels overall rising but not very fast at all (and just wait till they start meeting level-drainers!), lots of retirements, lots of death, lots of fun.
Single-party (I call them "linear") games/campaigns really are a different breed of animal, and are - unfortunately, IMO - much more common now than the multi-party campaigns. The style of play, for one thing, is much different. In a multi-party game, one can easily have one's character find a good reason to leave a party it doesn't want to be with, and then (usually with some DM co-operation) find a way to have it join one of the other active parties. The only difference for the player is she has to show up on a different night of the week.

The game is able to be more chaotic. In a single-party game, you either have to abide by the wishes of the party (or the party bully, depending on the characterizations being played) or leave. The game tends to be more lawful.
Multi-party games do require a DM with more than one night a week available for play; that alone is probably why they're less common than during the 1e days.
Lan-"which party am I in this week?"-efan