So the YED can get rid of Daddy Winchester by trading lives, but still, he could have lied about it and just let Dean die too.

Exactly! It doesn't make sense to have Dean around if he isn't important. Before 221 I was of the thought that the only reason the YED left Dean alive was so that he could protect Sam from the Hunters. However, seeing as the YED (I assume it was him) took down the Roadhouse with no problems, and how he could have taken Sam and put him somewhere else at any point, that line of thought pretty much went out of the window. One the things that have been going through my mind though is that maybe the YED took John's soul because he need it for some other purpose (a really good General is really hard to come by).

I have much hope that Kripke co are smart enough about the symbols and myths, and missing the opportunity to mess with the firstborn just seems strange. Also, one of the things I notice about SPN is that they actually PLAN all the major events. Revelations about the plot or the characterisation always feel inevitable. The writers lay the ground work for all major developments.

Thanks for reading!