Oh yes... beefing-up the numbers of your own troops performing a "threat-in-being" mission somewhere... maybe I shall add a Reason No.6 to what I had written above...
Still, IMHO the mercs only pay their rent fighting, not standing idly. Therefore, they should be somewhere beating the nine rings of hell out of some wretches. Besides, continual putting them in harm's way let you cut on their salary in the process.
Oh well, playing Mercia in VI you are probably swimming in florins already, so what the heck...