I suspect it does need one. As a buyer i'd want to closely examine a photo before paying for an EL or high value SOD and so on. Other agencies seem to have it.
However, its very hard to secure. If someone is determined enough if they can view it on a screen they can steal it (screen shot button etc).
i like the idea of a magnifyer zoom, ie. u can only zoom in on a specific area. a buyer like u say, can then inspect for noise or fringe at the area that is usually prone . i don't like the idea of what say, Flickr has , download in all sizes. but many on Flickr has no qualms of giving away their hard-work, it's their choice.
but yes, right click disengaged does not prevent a ctrl c . still, as one of my peers pointed out many years ago , 600*900 px should be the largest u give away on your site. sure, it runs the risk of being used in blogs,etc... but at least it is pretty much useless for even an 8by10. you can print it, but it won't be much good for a commercial print. not fail-safe, but then again, as we already know,
if someone wants to steal your work, they can and will find a way.
anyway, they r more likely to ripoff Yuri, or Mr. Locke's work, then little ole kookoo cher choo me
so, just not be too famous !!!