The Ubuntu SDK QML page http://developer.ubuntu.com/apps/qml/overview/ gives instruction to install the SDK from https://launchpad.net/~ubuntu-sdk-team/+archive/ppa which states:
This PPA provides Qt 5, Ubuntu UI Toolkit, Qt Creator and more for
various Ubuntu versions. It also provides some Qt modules that are not
part of Qt 5 - qtsensors, qt3d, qtlocation, qtfeedback,
qtconnectivity, qtsystems, qtpim, qtwayland. Those modules are just
snapshots and prone to any changes, but may be used for testing.
IMHO if a framework/library provides the/some functionality you need and you think it will save development effort without any obvious side effects (maintainability, portability, performance, ease of install on target platforms) then it is logical to use a framework.
Mark Shuttleworth's (pre touch) stance on frameworks http://www.markshuttleworth.com/archives/568 :
Ease of use, and effective integration, are key values in our user
experience. We care that the applications we choose are harmonious
with one another and the system as a whole. Historically, that has
meant that we’ve given very strong preference to applications written
using Gtk, because a certain amount of harmony comes by default from
the use of the same developer toolkit. That said, with OpenOffice and
Firefox having been there from the start, Gtk is clearly not an
absolute requirement. What I’m arguing now is that it’s the values
which are important, and the toolkit is only a means to that end. We
should evaluate apps on the basis of how well they meet the
requirement, not prejudice them on the basis of technical choices made
by the developer.