I usually make use of 1 single unit og Kataphraktoi. I mostly keep them in reserve, as they are quite valuable to me (AOR in not that wide, especially if you are fighting in the West or South). I mostly use them to counter heavy cavalry, pinning FM's, then flanking them with lighter cavalry or infantry. I als use them as a hammer or side-sweeper when I'm absolutely sure here is no way my enemy will be able to disturb them.

If you chose to field a nomad-style army, I suppose their usages differ. Maybe use them when you arows are spent or something like that. I don't really have any experience with any other army type except greek ones :P

Maion