I never asked this question and never read explanation, but I think it's quite obvious: You will inevitably hit icing conditions during descent in clouds, and you don't know when exactly. So, keep it on and you will never miss it.
That's what I thought for years, but it's not logical: otherwhise the same would count for climbing in clouds. Eventually you will reach 10 degrees, you just don't know when, so EAI on for the whole climb in clouds? Nah...
