I seperate out the PM units into full experienced units (who, if they are below strength, I fill out with less experienced troops just to give myself full units) and the less experienced 'dreggs'.

The elite go on garrison duty on city I feel are vital to my factions strength (but only a few units do this) and city on my more exciting frontiers to hold the line if my attacking armies suffer a disaster.

The dreggs get treated as a mercenary army, they march to war
Any units that gain the experience to be regarded as elite get brought back and used as stated above, the rest just fight battles after battle till they are annailated.