Those kinds of sellers in my eyes aren't of much help to me because after the initial sale, my product is easy to give away.
Don't kid yourself... your product is just as easy to "give away" regardless of who's doing the selling. The various DRM schemes can be simpler or more complicated, but they've all
been broken... with the exception of Apple's Fairplay DRM (which I'm sure would change if iBooks were to suddenly become more competitive in the ebook game). So if security to keep your product "safe" from being given away is your number one priority, you'd be better served going exclusively through iBooks. It's just not possible (at the moment) to bypass their DRM. Just remember that obscurity is much more damaging to an author's chances than casual sharing ever could be.