Holding a lord hostage to control his army isn't weak, it's smart.

If he had let the guy go free, that would be weak.

Even if his people did see it as weakness, at least he still had a full army. It might have even been seen as mercy. Of course, there might be negative effects in the hostage scenario as well, such as some groups losing trust in their leader, but it would still be better than losing half his army.