The reasons to join a pay per click program are many, but generally speaking it's excellent if you are running a site with many visitors, perhaps of a general nature.

If you are attracting visitors of low age, the chance of having them buy products you advertise are slim. The simple reason if because they don't have any means of buying online, often to young to have a credit card.

Simply put, if you have visitors unlikely to buy online, you might be better off with the PPC option.

If you are using pay per sale affiliate programs today, why not try an pay per click option as well? The worst that can happen is you loose a few minutes of your time to join the program, if it doesnt convert well enough - you can always go back to your PPS model.