Opera announcing their transition:

Consumers will initially notice better site compatibility, especially with mobile-facing sites - many of which have only been tested in WebKit browsers.

Site compatibility is something Opera has struggled with—last year they took the controversial decision to add support for the webkit vendor prefix, along with their own prefix, which undoubtedly introduced some confusion, but more importantly risked undermining the purpose of vendor prefixes. Craig Buckler wrote for SitePoint:

Taking the decision to it’s logical extreme, all vendors would support every prefix but any implementation differences would render the CSS property useless.

Although competition is healthy, Opera’s transition is promising for Opera users as well as developers; Opera users will experience a less broken browsing experience, while we’re avoiding the risk of further fragmenting the web. And hopefully, we’ll see WebKit grow even stronger. Opera, near the end of their announcement:

In the last few weeks we’ve contacted the Webkit project, and contributing organisations, to discuss our intentions to work with them to make WebKit even better. By contributing patches back to WebKit, we’ll enhance standards compliance across a range of browsers, not just Opera.