@Carbo, @Dorsai6 and @0d1n, thanks for your insights.
I agree with you, and think that the demos' expiration is bad for the loyal customer, and against Totem's purposes too, because it will get many of us buying way less cards.
That is particularly true for older cards, and I'll give an example about discounts too.
In september Totem offered many discounted cards. Many of them were cards whose demos I already had in my device; it took me some time to watch again the previews, but in the end... I bought the most of them. With expired previews, instead, I probably wouldn't have bought any of those cards.
Another point is that, rather obviously, the most people abusing the preview system are among the free users.
So, I still am convinced that such limits should apply just to new users and low level members.
My own habit, as a Platinum one, is: the more demos I can watch, and compare, and store on my device, and watch again later, the more cards I buy.