Hey Scott,
Primer is different color (sorry on my english... dont know how to explain better butt with word different) and how I know he is pritty ok telling you about it.
Before Testors primer I have used
Revell acrylic Stone Gray, 36175 and also with it get good results.
If is all ok with your surface you really dont need it except this ordinary color, but if you have some visible joint, "holes" scratchec... primer will cover them because primer bite plastic a little and color goes into the holes, scratches, etc...
If you dont get primer for that kind of holes, I have work like that before... if I had seen them, I took WHITE tamya putty (it's mutch thinly than GREY
tamiya putty) and cover those holes... and of course you have then again to sand and paint with these stone gray (or whatever color you choose).
In my knowlage, thats the difference
Cheers
Roman
P.S.
I have heard from my friend, he is using alclad primer... and he told me that alclad have mutch higher bite to the surface and for his oppinion it's the best primer