You're right, he doesn't know he would be cursed with no prior intel. If he was caught with all hearts in his body (beginning of fight) he might lose if danzo could finish him off quickly. If they are already separate, The other hearts would interfere and protect Kakuzu, they would not just sit idly by if he was paralyzed so danzo would need to fight them. The seal is also breakable, sasuke broke out with his chakra. Meaning that while Danzo is being held off by the other hearts for a short duration, Kakuzu could break free. this is written in the fourth databook:
Similar to the Cursed Tongue Eradication Seal, this seal can also be applied during combat, and can secretly be placed on the opponent's body. When the user activates it, the curse seal's marks spread around the opponent's body, paralysing them. However, one can break free from the seal with a strong enough release of chakra.
This seal is not an instant win for Danzo vs an opponent like Kakuzu because of his ability to have 4 other bodies to defend the main one.