Back of envelope math: $60 massage, $50 with coupon, $45 revenue for merchant. Works outstandingly well -- even before you account for the inevitable breakage.
However, I see a couple of potential issues with this approach.
1) Merchants can offer probably a 5 - 10 % discount (acquisition costs shifted to discounts), but beyond that I doubt if can be sustained. Margins in most local businesses are not that big. So, it'd be hard for a restaurant to do a 50% discounted deal for like a month or so.
2) Another potential problem of this approach - how many users will check each individual merchants store ? People flock to Groupon because there's a new deal every day. Or in other words, the novelty factor keeps things fresh and keeps people coming back. If a merchant is offering the same deal for 6 months via the store, it might not be a big draw for users. This will be completely contrary to the notion of scarcity and novelty that make groupon's a deal a day model so successful.