2. The Maxil Principle
As the Twelfth Doctor's casting process, and indeed the Sixth's, has shown us, appearing on 'Who' in another role does no harm to your chances of one day pocketing a TARDIS key yourself. Did you spot our erstwhile hero's cameos in ' The Eleventh Hour' & ' The Power Of Three'? Small, but perfectly formed and no doubt a dream come true for him and anyone else who's been asked to appear & accepted across 50 years of making the impossible possible. Plus the BBC would save a fortune on scientific researchers, a fact that most likely wouldn't escape the pencil-pushers or the drama department while there's a recession on & all.