Usually, The only nobles without unit are Traders, Priest and Infiltrators and maybe some diplomat. The thing is all this club-class but infiltrators are public, so this not make very easy to be 'in the shadows'. You have a big sign with an pointing arrow: 'Probable Infiltrator'.
I think it could be useful for Infiltrators have their sub-class not hidden, but changed to one of the others unit-less class... The infiltrator would chose if he want to be seen as a plain warrior or as a trader, diplomat, or priest...