You've got to watch out that you don't use a voucher that has a higher value than the cost of any night in a multi night booking. If you use a free night voucher, it will apply only to one night and I try to make sure that my vouchers are always a bit less than the cost of one night. Otherwise, you lose any change you thought you might have got back for your voucher. I usually find the discount, for discount is what it is, well worth the effort. It saves me a bit here and there, when I need it.
I click through the German cashback site shoop.de and you get 7% cashback if you don't use hotels.com Rewards, otherwise 5% if you do. I find that reasonable.