I do not agree with dismal ...

While the main idea behind the TW series is tactical combat and the way to get there, having agents at your disposition is a way to engage in the most favorable position...

To further your 3 points :

1 Muster army (I'd say Muster the army towards a precise objective) : to do this effective use of agents is necessary in that it permits you to muster only the troops necessary to your objective (gain of cash, of time...)

2 Attack enemy city : Can't fault you on that... But having spies can further change the type of army you will have to field, ie less artillery and less rams or towers, doors having a chance of being already open... thus further economy both in time and cash and thus the possibility of choosing a new objective faster...

3 Repeat : Exactly but with agents you should be ableto do it more often and with less cash spent

So, IMO, even in your style of play and even w/o micromanagement of agents, use of agents is a means of achiving your goals much more rapidly...