All the multi-flexi tickets could ever be is pre-paid unreserved seat. Nothing more. And if somebody gets to that seat and pays cash tough on you sit elsewhere or be locked out. Nothing else would work on a practical basis.
So the flexi ticket is just a way of giving Saints a bit of money a bit earlier and maybe get a free game thrown in. Nothing could be 'reserved' seating for you.
I can see the pluses from Saints point of view and it might make a nice christmas present for the more occasional fan but I can't see it being a big seller.
Nevertheless still a good idea.