If you're going to use your Groupon for a movie and a soda for $6 it's a great deal. If you don't make it in and the Groupon expires...you're screwed because SMG violates Groupon's policy.
It clearly states it can be used towards any goods or services after the promotional value has expired, if you contact Groupon they will tell you the exact same thing. If SMG refuses to accept your expired voucher for ANY GOODS OR SERVICES, contact Groupon and they will refund your money.