I prime some parts and not others, it all depends on what the part is.
I mainly prime parts that have had seams filled and so forth as the primer will show what work still needs to be done. The nature of the primer's pigment also allows it to fill in minor flaws which it covers up very well many times.
The best thing about primers is that they will grip the plastic very well if you use a solvent based one and they dry with a slight "tooth" which is a slightly rough surface which allows subsequent paint over the primer to adhere better.
Another time to prime is when the plastic is a dark color and you want to paint it a light color such as white or yellow. The primer makes a good base for that also.
I personally use only one primer and that is Floquil Primer from their model railroad line of paints. It is smooth, flows very well out of the airbrush and always produces a good base for me.
Mike
“Wisdom is the right use of knowledge. To know is not
to be wise. Many men know a great deal, and are all the greater fools
for it. There is no fool so great a fool as a knowing fool. But to know
how to use knowledge is to have wisdom. " Charles Spurgeon