While I admit the uses for it will be few, I'd just prefer not to have to message my friend whenever I have something for myself. I check the forums every morning, so it would be the most likely place I'd notice it.
I see nothing wrong with old-fashioned paper and pen/pencil, computer documents, etc. That is what the majority of people do, because it is reliable and has more customization and options. (As well, it has been proven that when you hand write something, it stays better in your memory.) As for starting a conversation/PM with yourself, it may not be possible with Xenforo. If memory serves me correct, most forums allow this, but if Xenforo did, it would have been default. Not sure why Andrewkm or KHobbits would disable it, so I am going to assume it isn't possible. That, or they wanted to follow suit with in-game, where you can no longer PM yourself. If it is possible, and it wasn't disabled due to whatever issue/reason there is or was before, then I see no issue with enabling it. However, I will personally stick to paper and pen/pencil, as it has more options (that and I adore stickynotes, lol).
I'm going to have to agree with Moofink on this one. I have no reason to forum PM myself. If I have a note I keep it in my TextEdit aka NotePad for Macs. I have a file called "Important ECC Information' Thats where I keep anything in regards to ECC I think I need to know and I keep it open to reference. If you feel thats too much/not enough then use word/excel/ etc.
But you can still mail yourself, and you can still send yourself money. If it's not possible that's one thing but, otherwise I would prefer just to use a forum conversation then leave some piece of paper around for someone to pick up and go "What in the world is this."
There is really no point to this to be honest, why waste forum inbox space whereas most people write it down on Skype, paper, Keep, or any note taking app, I always just write it down on my phone.
Because you can't do it on ECC, and there's no need for it to be possible on ECC exactly because there are a lot of things that can do it. Not to mention this is a forum software, it's not meant for notekeeping.