Title says all, but I'll give an example. So you just logged on to the server and you don't want anyone messaging you and their spamming you, I know there's a ignore command but if /togglepm was added you wouldn't have to worry about spammers (wurst has a spammer option). And if they get on multiple accounts you'd get pretty tired of /ignore and their name, and they keep coming back on alts and spamming. I know this is a very low chance of happening but I'm not going to explain anymore. Yes mods will be able to bypass.
Neutral. Because essentials doesn't have that kind of function there would need to be some kind of plugin to compensate or make this happen. However there are chat mods that allow you to sort out your chats and things which could be very useful. I don't think pm gets intrusive unless there is someone spamming and that can be easily dealt with
No support. This would just give the developers more work to do, and I do not think it would be very useful. I highly doubt that anyone would get on multiple accounts just to spam your private messages. I think that private messaging is useful, and I would find it annoying if I couldn't private message people because they forgot to turn it back on. Overall, I do not think it will help, and you can always use /ignore.
