This is definetly one way to improve hero leveling. Esspecially if you have some low security cities nearby or some prime targets you are plotting to take soon. You can use the vision it provides to scout enemy defenses, and possibly even risk using the moral or fortification debuffs right as your army is closing and you declare war. Just be aware you may injure your hero by taking an action but it may be worth the risk. 

It's definitely a good idea, since other empires may need to do round-ups in order to catch your spies, which lowers their production output by 50%.

So either way, I think that you get quite a bit from the dust you've spent for hiring the hero.

Redsang should also double the XP your heroes get.

If a hero is leading an army with 5+ units, he is getting more XP even when not fighting anything. Still a cool idea, although I rarely find that there is nothing to fight.

Generals get 1 XP per turn for each unit in their army.  So a general with a 6 unit army will get 7 per turn.  1 base plus 6 for the units.  City governors also get extra XP per turn for units in their city garrison (not counting militia) at the same rate.

 So for generals, you will probably get more XP per turn with a full army than using them as espionage, without the risk of them getting captured or wasting influence implanting a spy you never plan on using.