Probably using the definition of conceit where it means a far fetched idea or metaphor.

2.something that is conceived in the mind; a thought; idea: He jotted down the conceits of his idle hours. ...
5.an elaborate, fanciful metaphor, esp. of a strained or far-fetched nature.

taken from: http://dictionary.reference.com/browse/conceit

Makes perfect sense to me; the idea that you meet a lot of people is far fetched, but makes the game play work, so you use your imagination to pretend it isn't unrealistic...

Shakespeare got a lot of mileage out of variations on 'a bunch of stuff happens... then... everybody dies' - so I'm not sure what your objection is.

I've read most of Brunner's work. Someone survives in nearly all of them though, so probably not the same ones radtea read :)