This Qt update breaks some functionality in Kontact & Korganizer - and possibly other apps that depends on kdepim. Packages may not move to main that break existing packages unless working updates for those packages are also moved. AFAIK, nobody has gotten around to building an updated kdepim.
On the other hand, there are some new packages that need Qt 4.6.3 to build, so for now it stays in testing for those that need it.
If you don't use Kontact or Korganizer you likely won't experience any issues installing this update.