The use case for setProcessEventsInterval() (http://doc.trolltech.com/4.7/qscriptengine.html#setProcessEventsInterval) is to keep the GUI responsive when evaluating a potentially long-running script in the GUI thread.
The main issue with this API is that it's too fine-grained. It's very hard (impossible, in some cases) to guarantee that we will call processEvents() at the requested interval – especially when native functions (of which we have no control of the running time) are called from JS.
The replacement API should have a more general notion of a timeout (i.e. not tied to milliseconds).