Hi there.
Well done for playing around and coming up with this for yourself. However, I'm afraid this does not strike me as strong magic and given the multitude of very simple methods that could be used to achieve this effect, there is clearly a reason for this not being commonly used.
When you consider that the effect is ultimately really just knowing the spectators card, the premise of using mates isn't so well suited here. Why go through and find the mate when all you are demonstrating is that you know their card? Many routines already exist where a mate is used as a prediction, and the spectator somehow selects the mate of the prediction, and in these cases, the use of a mate is justified.
If you think about this from the spectators point of view, the only thing that makes this different to having them pick a card and you naming it is the fact that you pull out a mate. This doesn't really add anything to the effect of revealing their card with a straight revelation.
I suggest you check out Allan Ackerman's "Another Quick Coincidence", which is a very good version of such an effect. The magician starts by removing a card as a prediction, then has a spectator select a card. The spectator slides their selection face up anywhere they like in the deck, leaving it outjogged. The magician then reveals his prediction to be the mate of the selection, then he goes one further to show that the cards either side of where the spectator slid their selection complete the four of a kind. It's taught on Ammar's Easy to Master Card Miracles Volume 5.
Joe
