Load Control - Each ap balances in isolation from the other aps. It's different than the Load Group setting that balance client loads between each ap within the Load Group. If you set a limit of users per radio and that limit is met, the radio will essentially ignore a new request which should force the user to contact another access point close by. If the client is adamant about attaching to the ap with the limit set, the radio will eventually let it on.
If you choose the strict setting along with a max number and that max number is met, any client that is above the max number will fail to connect to the ap.
So to answer your question, no you don't need Load Control for Radio Preference to work.
Doug Hyde Director, Technical Support / Extreme Networks