Yes, it's just about having enough Thieves so that 50 Dragons join you. <- The point is: the strength of an army depends on the number of units in it, and Thieves are weak in combat. You can safely engage them with a starting army (provided that their number on the map is set by the computer, not by a person) at any difficulty level.
Sincerely, Draco
In my opinion, the chance is quite low... and most importantly, Thieves don't help with recruiting... but it's a hassle to figure out how many you need...
It's a different story if a secondary hero is running around with these Thieves and occasionally transfers them to the main hero. But that's also a lot of trouble...
I wonder how often people use this tactic? And how often do they "ignore" these Thieves?
I "ignore" the Thieves if there is no Diplomacy skill. If there is, then only if I have the Scouting spell.