I feel that users who have purchases the map editor should be able to trade maps with other users whom have also purchased the map editor feature. This i think would promote many players wanting to purchase the editor not to mention if you play a map you really like someone made you could simply save it to your 10 slots.
That's a possibility, but I think if you like a certain map, you can ask them for a screenshot or something and have them post it on the forums so they can just copy it themselves.
Halo has an option where you can save custom gametypes and maps that you recently played, and I personally love this feature. We could certainly add a button/option in the game lobby and/or menu that allows you to save the map to one of your available slots. I think this would add further value to purchasing map editor slots.