I think it's a great idea to get the fans through the gate and hopefully get them interested in returning when Utd/City aren't playing at other times, at full price.
I don't know how many take up the offer but if we get 10-20 back for other games, it's another £100- £200, plus whatever they may spend whilst in the ground, in the coffers.
If it does become a regular thing, I agree that there has to be some kind of 'reward' for Alty season ticket holders.
If it becomes too regular, it will be along the lines of a DFS 'sale' and it'll just turn into the norm, which is not what those going week in week out, without the discounted entrance, will want to see.
Great as a promotion, not so great as a regular occurrence.