Yes, that would be useful. Problem is, it's a bear to collect that kind of information. Even if hotel reservations agents, front-desk personnel, and/or concierges cooperate (many don't), calling 100 hotel-casinos and asking them to list all the coupons their room guests receive would take a month -- at which point, it would be time to start all over again.
Also, in our experience over the past few years, the coupons aren't especially valuable (imagine that): a two-fer drink, small-percentage discounts at the buffet/fast-food eateries, 2-for-1 last-minute tickets to the shows, maybe some special meal deal in the steakhouse, 10%-15% off in a retail shop or two.
Finally, as you suspect, these coupons do change quite a bit. The more valuable coupons/promotions are often short-term offers that, as soon as we unearthed them, would be on their way to obsolescence.
How we wish the casinos would make our job easier. Unfortunately, these kinds of ambitions are just too much of a struggle. Sorry, PJ.