From my experience, newer technologies make building software, simpler, quicker, less error prone, and more feature rich. Therefore, when a new subsystem or fundamental paradigm becomes available, the developer must take a good look at what that change will do to their software product. In many cases, the maintainability and performance of migration are reward enough for the effort, but even when they are not, the developer needs to look towards future requirements, and the difference in how they would be supported under the old and new subsystems. It's my philosophy to upgrade to new frameworks (such as in this case Qt3 to Qt4) as soon as they are stable, and at times, even delay the implementation of new features while awaiting the release of such a new framework, as a general rule.
As I already mentioned, new frameworks often make things easier to do, more stable once their built, and easier to understand and maintain. If a team were to ignore such innovations, and plow headlong into features without considering the new framework, they'll find themselves with an obsolete codebase sooner than they think, as the updated framework makes much of the work they have done redundant. I furthermore embrace the update of said frameworks as an opportunity to challenge existing assumptions, perform a rather detailed review of the system as it stands, as well as architecturally plan for future needs--essentially as an opportunity for a large scale refactoring. Even in "cathedral" style projects, requirements change, technology changes, people change, and updated frameworks are a great driver to readdress the architectural and quality concerns of the system.
I feel that the bazaar type project falls in the middle of the spectrum in terms of efficiency and output quality, as compared to well managed (and understood) cathedral projects, and poorly managed cathedral projects. The Bazaar certainly has the advantage over the poorly managed cathedral, as the workers can stop working on a portion of the software that they are certain are doomed to fail, for whatever reason, and can redirect effort elsewhere. This freedom to do so, however, would be detrimental in a well managed cathedral project, where perhaps its hard to communicate the full scope and applicability of a portion of the software to the developers. A certain portion of the application may fill great needs for HR and Accounting personnel, but if the programmers aren't interested in these fields, and therefore don't see the need, this portion of the user base would be alienated. In this manner, a (well managed) cathedral project manager could better allocate resources than an open project would, by simply working on what is interesting.
Overall, my feeling is that a bazaar structure works very well when the user base is very well in line with the developer base, and that the total number of hours worked is not of prime concern. From here, I shall refer to the cathedral style meaning a well managed cathedral project. However, I feel that the cathedral style is much more effective when there are silent user bases, or maybe user bases that are sufficiently unskilled to foresee the use of the software to the extent that they fail to participate in its design. Cathedral style also benefits when there is concern with the productivity per developer hour spent (such as in a company desiring to minimize costs of constructing said software).
The real strengths of the bazaar is that anyone can contribute. This benefits the project when people with unique skills, perspectives, and goals join a project, and are then able to contribute in a way that the existing developer base would have been unable to.