At a rough estimate I'd guess I'll have spent something like 400.00 for the whole of 2015 by yearend. Assuming my spending rate stays about the same, a year from now, at non-VIP status, that would get me a book with a normal price of maybe 7.00 (which I'd usually get for around 5.00 with a coupon).
That would be a nice little bonus, but it wouldn't influence my normal buying decisions at all so Kobo would have all this extra record-keeping to do for no financial gain (at least from me).
Actually, since someone mentioned the VIP membership is couponable I went ahead and signed up for 6.50. There isn't enough information to make any sort of rational estimate of if it will be worth it but for that price I'll try it for a year --- so I guess if many other people are as impulsive as I am Kobo will get some money after all.
One practice I dislike is that they'll automatically charge you every year if you don't cancel. I've just added a reminder to my calendar so I'll remember to decide one way or the other.