Hi zfactor
As you have pointed out, the license model seems to be built upon some shaky assumptions.
Example:
Today I buy a 1-user, 3-year license, expiring 12/12/2013. I insert the file on one PC.
Tomorrow I buy a 5-user, 1-year license, expiring 13/12/2011. I receive the license file issued with the second purchase which also includes the info from the first. When I insert this file into the 5 PCs, all of them will indicate license valid until 12/12/2013.
People will very likely choose to use the one email address for all online purchases, be it for home, work, other family members, etc. As it is, it can't cope with concurrent license periods under the same primary key, in this case, your email address.