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Doesn't it all come down to the same thing?
If creatures join, they join. If they run away, they run away. If they attack, they attack...
Thieves don't affect the diplomacy skill in any way.
Best regards, Draco
Also, if monsters join for gold, you might be short 100 gold. Without thieves, you would simply attack them and kill them. But this way, you sell resources and gain a significant advantage in your army.