I think that's probably right - singletons are the anomaly - perhaps the inbox should only display items which don't have a context.

Nevertheless, I do think it would be useful if items directly entered under a project (not explicitly typed in QE or the inbox) were still to be displayed in the inbox (as well as in the project outline), if they didn't have a context.

In other words you don't prevent things from getting into the project list, but you also show them in the inbox until their context is processed.

This would build confidence in the inbox, making it a useful and predictable list of all incompletely processed items, rather than just one possible point of arrival.