While it works without qtwebkit, it doesn't render help pages propperly. It falls back to some text-browser like mode, which isn't very nice for browsing the help pages. Simply installing the dev-qt/qtwebkit:5 is enough to make it work, so qt-creator should depend on it.
IMO it's not a good idea to make a hard dependency if it's optional. qtwebkit is a massive package - and just to get more beautiful docs? pkg_postinst message or a new "recommended_deps" is the way to go.
Yeah I know about this, earlier versions are affected too. Unless it has changed in 3.3.0, it's a build-time optional dep (automagic too), so I'm going to add a webkit useflag for conditional compilation.
*** Bug 538430 has been marked as a duplicate of this bug. ***
Fixed in 3.2.2 and 3.4.0