There are reasonable limits on how often you can activate a given product to help deter piracy, but for most users, this limit will not be reached.
There is some flexibility to allow installing on your laptop and home PCs, home and work PCs, and similar situations.
For instance, you might want to use your laptop as a secondary machine with Multiplicity at home and at the office. This would require Multiplicity be activated twice (once on each of the primary machines). As long as you're the only one using those machines, that's fine and covered by your license. When you start getting more complicated like "I also need copies at client sites for when I'm there", then it's probably time to order some additional licenses!
Bluntly: If we find 42 activation attempts from 29 different IP addresses and emails in the course of an evening, it's fraud. We are users of our own software too. We've added a level of protection that we, as users, are comfortable working with.