I haven't been back to highlight the key points, but yes, these friends with benefits or cohabitation situationships are never as "easy come, easy go" as they're originally thought to be. They just set people up to be broken inside and therefore, making it harder for them to be in any healthy relationship.
They either get into a new relationship right away (in order to make up for the benefits they'll lose from the FWB) OR they start to cheat with the FWB as soon as the new relationship hits roadblocks.
If a person still keeps in contact with a FWB even though they claim that it's over, RUN. These people do not need to stay in touch if they don't have children together. They're keeping each other at arm's length for the convenience of physical and emotional companionship should they hit roadblocks in their current dating life.