In my single-player, Hard campaign game - I've found that Metsuke (the secret police of Shogun 2) agents are useful to boost income and catch enemy ninja. What confuses me is why I can't hire any more - the tool-tip indicates that it is based on the number of market-chain buildings I have, and I am sure I have more markets than Metsuke.
Also, it is not clear whether an upgrade to a market counts as an extra 'market-chain' building - I am guessing that it doesn't.