Primer and and finish coat paint have different characteristics.
Primer is intended to seal off the surface it's applied to and to
provide a surface for another coat to adhere well to. A finish coat
is designed to be durable, provide a surface that can hold up well to
occasional cleaning, physical contact, etc.
Given that it's a ceiling, it certainly would not be as bad as leaving
an exterior wall or an interior wall with primer, instead of regular
paint. But if it were my home, I'd put a coat of regular paint on it.
You can do whatever you want. The primer acts as a sealer and base for the
finish coat. It may yellow faster than the finish coat, it would be more
difficult to clean, but no one washes ceilings anyway. Finish coat may look
better if you compare the two side by side.
Since you already have things covered, furniture moved, etc, I'd go ahead
and put the ceiling paint on, but that is me, not you. The finished ceiling
will look OK for a longer time, and you may have to do the job again a few
years sooner. Your choice.
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