Interesting topic. I am definitely on the fence with this one concerning one option over the other. I think I would prefer a hybrid option.... let me explain
On the one hand, I hate not being sure of a pitch before I set off. I have a caravan so wild camping or pub stopovers are not options for us. I love Galway (as an example) but seriously, I would not set off on that journey without the security of knowing that there is a pitch available for me when I get there. As you referred to the 'first come, first served' basis - it would not work for us. So in the case of Galway, I'd love a booking system.
On the other hand, some sites take bookings without a deposit and tend to be booked up as soon as the booking engine is opened for the year. Still, if you rock up to the gate (purely taking a chance), there is always the possibility of some or even a few not turning up. The reason is that they block book sites because A). it allows them to decide where to go on any given weekend and b), it doesn't cost anything as there is no deposit involved.
So my feelings are that I would like a booking system for all sites but with a deposit system. That way, if you are going to book it and pay a deposit, the chances are that you are going to go and unless you are extremely flush with money, you are not going to book multiple sites with deposits to lose for those that you don't go to.