There's nothing wrong with borrowing aircraft from anywhere if it fills a capability gap that would otherwise remain. If CAS was overseeing the loan of U-2S aircraft to plug the gap left by the withdrawal of the PR9 I'd be outside Main Building with a magnum of champagne waiting to kiss him.
What's wrong is when the loan of an aircraft might result in some degradation of capability, or might erode our autonomous national capability in a certain (and vital) area, or where it will prevent the development and deployment of a better optimised, more suitable replacement.
It's complicated, of course, because Rivet Joint is superior to Nimrod R in some areas (including areas that are of particular relevance in this phase of the ongoing ops in Afghanistan and Iraq), while inferior in others, which might be more useful over the longer term.
The allegation is that proper priorities are being distorted by funding constraints and by a too narrow and too short term focus on Afghanistan and Iraq.