I think customers are more and more aware of this and are either a) viewing the deal provider and vendor with mistrust or b) shopping around and finding possibly a better price available (where a vendor hasn't had to pay another 30-50% of the sale price to the deal provider)
Edit - not having a go or criticism of gravity10 to be clear - just an observation
Agree 100% with you, a certain offer came up here a number of occasions recently, my partner is in the same business. Prices charged in her business are less than the so called deal price.
I can see deals working for service industry, they can sell additional services to you when you arrive, its a hook to get you in.
As for items that are delivered as one offs, I take them with a pinch of salt.