True, but vendors do have lock-in. If you get big fast enough and cover a metro area with in place agreements to couponize offerings that makes it harder for a new player to sell to the same business. Basically you're asking Joes Pizza Parlor and Nail Salon to send offers to two networks now. Unless Groupon completely screwed the relationship from the beginning this will be 'harder' (in the sense that small business doesn't really want to manage a bunch of vendors so there is natural resistance to taking on new ones).
That being said, it creates an opportunity for an 'offer market maker' which is to say a clearing house which can put your offer on various channels (via Groupon via CloneOn or whatever) offer enhanced analytics about how well it does and let you slosh around your limited advertising/marketing dollars.
Actually, this is why I think Square is so well positioned. They're offering a full suite of merchant services, from payment processing to loyalty program management to proximity advertising, and there's nothing stopping them from joining the coupon space if they were inclined to do so.
To lock in the vendor that doesn't want to deal with multiple companies, I think you're going to have to provide a lot more than occasional access to an email list.
That being said, it creates an opportunity for an 'offer market maker' which is to say a clearing house which can put your offer on various channels (via Groupon via CloneOn or whatever) offer enhanced analytics about how well it does and let you slosh around your limited advertising/marketing dollars.