Depending on the habitat, I think that these displays can be semi-beneficial. Some ray habitats I've seen are only so-so - bare bottomed, unnatural colors...probably not what stingrays would prefer. Others have had sandy bottoms and a more natural look that is more like their natural environments. I'm pretty sure stingrays would be more comfortable in a habitat like that.
As far as putting stress on the animals, I don't think it's TOO much of a problem, as long as the rays have a way to escape the people touching them. I don't totally agree with the petting marine animals displays, but kids (and adults) can learn a lot from these type of exhibits if they are done in the right way.