We allow our customers to install our software on as many machines as they want. However, we use a hardware key. Without the key, the software runs in demo mode. Plug the key in, and it's fully functional again. This allows them to buy one license, install it on many machines, and move the hardware key from machine to machine. If they wish to run two copies of the software on two machines at once, they need to buy another license, which gets them another key.
The key is expensive, cuts into our profit margin, and does occasionally cause installation problems all by itself. But our customers seem to like being able to buy one license, and use it on their desktop machine one day, their laptop another, and their own customers' computers the next.
It must be an odd approach, because we do get questioned on it, as if they didn't hear us right, and they can't believe what they just heard. "You mean I can install this software on more than one computer, and you don't care?" "Nope, I don't care. In fact, I encourage it."
Does "hardware key" = dongle? Just curious about the mechanism.
But, the dongle solution is far more maintainable than than scourge of SysAdmins everywhere, the reviled and hated FlexLM monstrosity.