An app developer will write an app using Qt and it will run on all Qt enabled devices (with a simple recompile if the arch changes). This means that the developers won't target specifically MeeGo 1 but Qt, giving them a much larger market.
The same app will run on the n900, the MeeGo based successors (and MeeGo based TV screens, toasters and coffee machines) as well as on newer Symbian phones.