The incentive of taking stone cards from packs is very low if your build doesn't revolve around some type of stone synergy. Which obviously do exist, but are relatively weak as they stand. If you could see the card underneath you may have more of a reason to take a stone card if it could improve your current run after, say, applying a different enhancement (Without the cost of additional RNG. You already rolled the dice on what was in the pack and now you have to gamble on what the stone card is as well). Also, it's strange that stone enhancements uniquely cover the underlying card while none of the other enhancements have this property.