@Jace -

Ok… but under the terms of the GPL, if I buy a copy and obtain your source I am then able to redistribute that source code to the public for free without charging a fee. That is to say, if I want to buy a license, I can then post the plugin to my Web site and allow the masses to download it for free… without every paying pluginbuddy or myself a dime, if i so choose.

Entirely correct.

This brings about two points… if not ok with that (and I don’t know why anyone who demands payment for binary code to obtain the source code would be ok with it… otherwise, they would just distribute it freely themselves),

...enter the spirit of the GPL plea. There are some WordPress Theme/Plugin developers who license their work under GPL, and then attempt, through persuasion/peer pressure/etc., to prevent you from exercising your rights due to that exercise being unethical. (To which I say: hogwash.)

...why choose a GPL license?

Bottom line? Because it is the community more/expectation/standard. The vast majority (if not the entirety) of commercial WordPress developers who release their work under GPL do so because they want to be on good terms with the WordPress community.

Furthermore, how do you actually license the software to be used on 2 sites, 10 sites, etc. If I purchase a license and then have the ability to redistribute the software freely, if I so choose, doesn’t 1 license in fact equal unlimited licenses as I can redistribute the software to each and every client I build a Web site for?

IIRC, the per-site license is for support and updates, not for mere use.