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What exactly do you mean? Be more specific. What does "got one, it has 600 or 800 health and high speed" mean – are you talking about a commander, a monster, or a hero?
VDV_forever gave me (and others) some helpful advice in one of his posts about hiring monsters in the Dungeon castle (in the portal) – that was also pretty good ;)
Sometimes it's very useful to know whether these monsters will join you or not. If they attack, there's a chance to increase the strength of your army and then recruit them. Without thieves, not only do the monsters not join, but you also lose your own troops.
Also, if monsters join for money, you might be short 100 gold. Without thieves, you would just attack them and kill them. But this way, you sell resources and gain a significant advantage in your army.