It would make my job as a mod, admin, or owner on my forum a lot easier if flamers/trolls use Personal Messages (PM) to troll and send dumb comments since I would not have to delete as many troll comments on multiple topics located in different sections of forums.
If trolls on Youtube send PM instead of use comments for videos, the video owner just have to delete and block PM from his inbox instead of going to individual videos to block and delete comments is a lot quicker. Plus, deleting PM can be done at anytime since they are not public, so they don't make your videos and channel profile look trolly or spammy.
Yes, I agree with you flamers and trolls should find a better hobby.
I think "Personal Message" function on forum software is a good way to debate with someone on a controversial "point of view" instead of having everyone have to read personal attacks directed at one or a small group of people on the forum since it can lead to more unfriendly messages by other members if the rude messages are not deleted.
But, I feel most people who harass online are too lazy or shy to harass by themselves, so they follow a group of people with similar "points of views".
Another reason to use "personal messages" is to share porn, and other content which violates the "Terms of Service" for InvisionFree, Jcink, Free Forums or "The rules of the forum made by the owner" or advertisng networks like Adsense, Adwords, Adbrite, etc since owners don't care what are in PMs.
Having your forum suspended and losing Advertisers(main source of income for many forums) because a member is posting porn or hate speech while you are sick or on holiday is frustrating.