Opera's precedent is opening the door for Mozilla (Firefox) and Microsoft (IE) to do the same, meaning that for authors—the group already accused of being too lazy to do the right thing—the Web is about to become an ever-more fragmented mess, where the once-safe experimental feature known as -webkit- (and -moz- etc.) now will come to represent a much more fragile, questionable feature or implementation.
This is a terrible decision for the Web.