As a "Gold" I've gotten upgrade certificates in the mail several years in a row. I've never used them because:
1. They cannot be combined with any discount or promotion. I generally use either a discount code or are using the weekend rate.
2. I normally get a free upgrade anyhow since the staff at my local location knows I'm a regular. When I'm out of town, I usually get a free upgrade anyhow, I suspect because I'm a Gold.
Bottom line is the coupons are basically useless to me. That said, I haven't gotten my "next year" coupons yet.