spies are excellent and have at least 3 good uses apart from being good archers on the field.
1 Spy on villages and towns plus raising their loyalty, rught click on town and get the option to send a spy if you have one in party. These spies can also engage in sabotage once they are settled in. You cna check their reports in the reports menu.
2 Send messages to other lords, right click on them to get that option if you have a spy in party. I never use this but I hear you can eve nsend them gisfts this way to impress them.
3 skirmish parties of enemies. Right click on a party, choose skirmish if you have at least 5 spies.The spies will chase them and slow them down so you may catch up with your main force. Be careful, the spies will in turn be chased by other enemy parties and if you lose sight of them you will not be able to send out other groups of skirmishers.
The cart is not that useful because you usually cannot chose a nearby city to send it to, the cart requires guarding and once it is lost it is lost for good. THe cart will slow you down but also provide lots of cargo space. If you must send it out on a trade run, give it 80+ troops so no one save enemy lords will dare attack it. The cart can be a good place to keep troops off the payroll.
Cheers,
Gerard