I think there is a common misconception here.
Let's be honest.
You don't actually gain knowledge by building LFS (copy/paste is all you do at your first build or even the second one)

,but is when you finally decide to maintain an LFS system,which is a pretty hard at the beggining,i have to confess.
Then you have to face the cruel reality,because suddenly you realize that you can't rely to portage to build the system for you,or you have noone to depend on him to update the packages,so what you do?
1.You have to write your own scripts.That's a gain,considerable maybe but not an important one (I wrote my own bash scripts -quite effective to be honest - but that doesn't make me a bash expert,ok).
2.You have to learn the use of every single package,to grep the Makefiles for dependencies and to choose what you actually want,to read the changelogs etc... but things that everyone who is willing to learn,can do in every distribution (oh well in almost every distribution)

3.Then you have to find ways to monitor the packages for updates,but you can't really consider it like an educational task,can you?
So what is the reason i am staying in LFS for more than one year now?
Although unquestionable gentoo is the most flexible distribution out there (USE FLAGS),LFS clearly wins in that regard (although i am always considering LFS most like a research project,rather as a distribution), for the simple reason that the choice is all and *only* yours (portage can be a really pain sometimes,especially when you mix stable/unstable branches).
Common LFS moto:Your distro,Your rules.
The development process is rather quick,especially in the development Books (they are in gcc-4* series since the middle of summer),and without a simple breakage so far,in almost 20 builds in various branches I made.
And there is this very personal reason.
I like the L(B)FS croud,editors (quite knowledgeable) and userbase.
Sometimes friendly,sometimes extreme and rude (well sometimes),but always enjoyable and to the point.
To sum it up,you don't gain more knowledge doing LFS more than gentoo already gives you,is just a personal preference,i guess and nothing more.