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Most of what you discussed was how to stick parties into the middle of a
game. But it's just as useful, if not more so, to start off with one.
(Maybe you hinted at this and I missed it.) If there's a bunch of
characters who have a common contact, however distant, a party may just
be the perfect kickoff point for a campaign:

1) It brings everyone together conveniently enough that a bad guy could
crash the party, drawing them into the main conflict.
2) Dozens of future plot hooks can be introduced. (Kind of like the
Millennial Fair at the beginning of Chrono Trigger, if you've played
that.)
3) Many characters (Player or otherwise) could get drunk and briefly
show off their true colors. 

A good GM could make these into plot hooks themselves. And nothing
strengthens friendship bonds quite like parties...

A party at the beginning of a game might also be an invaluable character
creation tool. For example, if someone has no idea what they want their
character to be, throw them into a party. Have various people at the
party come ask her questions, and use her answers to create a character.
For example, if the local priest comes and talks to her, will she act
shifty, avoid him, ignore him, or chat like old friends? Another
advantage (I'm performing a thought experiment here, so I may be wrong)
could be that these responses may be very natural to the player, making
playing the PC in-character much easier to the player.