On the contrary you'll find countless posts which recommend you prime all faces of ply, including the undersides which you are not fixing to. This is to combat warping of the ply by atmospheric moisture (if you've ever seem cupped of bowed timber worktops you'll understand how even the smallest of difference of moisturebetween the top and bottom of timber can have a devastating effect)
If you've seen posts where priming is not recommended you need to see in what context that is being suggested and also the products used to adhere to it.