Be daft not to offer a cross grade or upgrade.
I tell you if I had the cash I would rip their hands off for that. I know some will look scornfully at me for it, but meh.
I'd happily go along on my own and do the rides all day. I enjoy the social days, and the quiet ones, question is though, if you can go that often (especially on quieter days) is it really worth it? Last year was an anomaly with Smiler opening so late, I remember Th13455teen being busy it's opening season with the issues it caused too. I got quite annoyed I could never get there on a quiet day.
It makes sense if you can only get there on weekends but like to go often and don't mind going on your own.
I think the quantity they released will reflect that across their park range, especially Thorpe with London money knocking around.
I just hope that they factor these passes in when they set the volume sold on a daily basis (though I doubt it).
Also, I will be very surprised if Merlin do not allow you to have your Birthday vouchers inside the season opening. It is in their best interests to bring visitors through the door, we all know revenue inside the park is becoming ever more important rather than gate.
Personally I think it is a cracking little idea, and works well for everyone, I certainly have no qualms with businesses winning customers by providing a better value product or enticement. That is precisely how business should be run. Great idea from them and a wonderful little perk.