I want to create a premium membership with certain functions reserved to only those members. That's why. For example, the function could be turned on but limited to only two entries. If you want to be able to watch more threads, you would have to upgrade to get that any many other functions that together adds a real value to the experience.
Interesting.... however I'm not so sure that limiting what most consider a vital forum basic function is going to make you very popular. I don't think it will do aqnything for activity levels either, however, who am I to judge. Maybe your community is different.
No offence this is just my personal opinion but I think you would be disabling this feature shortly after paying to have it developed.