It doesn't appear that there is any way to actually book a site through thedyrt.com, it just seems like a collection of campgrounds and reviews.
The Pro version only seems to give you an offline app so you can find campgrounds when you don't have internet access, discounts on participating campgrounds, and discounts at camping and gear retailers.
It could be worth it if the campgrounds you like to camp at offer a discount to pro members. Here's a list of participating
campgrounds.
Edited to add: there is no mention of priority access to sites for members. The only thing I found regarding reservations is "Save 40% on your first night (Sun-Thurs, 2-night minimum, holidays excluded), and 10% off all other nights at campgrounds offering the Premium Discount."
-Michael