Having plenty of vacuum for the power brakes is nice. AC cars also utilize vacuum for actuating the doors/diverters in the system that channel the airflow from one place to another, but I don't know how much vacuum you actually need for that. idle vac. might also be important to someone running manifold vacuum for distributor advance.
Those are specifics. In the larger scheme of things it's about low speed, off-idle driveability and response, how tractable the engine is in around-town driving. Many, maybe even most "street" car guys want an engine that starts easy, doesn't take forever and a day to warm up, and runs relatively efficiently and has good power right from the stoplight without having to run real loose converters or dealing with an engine that wants to buck or stall in a manual trans car when maneuvering in a parking lot or driving in stop & go traffic.
IME, Anything less than about 12-14" of vacuum at idle and the brakes start getting wonky.
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'72 Formula 400 Lucerne Blue, Blue Deluxe interior - My first car!
'73 Firebird 350/4-speed Black on Black, mix & match.
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