The companion reward voucher can only be used when paying for a full fare seat yourself. This unfortuantely renders the voucher useless to most people.

From what I can gather, to use it on an UC redemption you would have to book a full J fare for yourself, but if you wanted to use it for PE or E there are actually a few lower fare buckets that are valid, such as both W and S fares for PE. Often a PE S fare is not that much more that a fully discounted PE fare, and far cheaper than a W fare which is fully flex PE.

Also there needs to be reward seats available to book the companion into, and in many cases if there was reward availability then you wouldnt be buying a revenue ticket anyway as you could already be using miles to travel.