Interesting, seems that many a ready to imply that it is not really possible to know more than one field well. Why is it so? Maybe I'd agree if we were considering totally unrelated fields, but that's not the case.
I consider myself full-stack+ developer, meaning that I can design and implement backend (RoR, PHP), deploy it, make a frontend (including mobile/responsive, whatever) and make an iOS app using that said backend (btw, drop me a line if anyone needs this ;).
I'd gladly let setting up production servers and doing graphical design being done by someone better skilled at these areas than me, but the rest would be pretty solid and you wouldn't get better results if "specialist" would do that. I do however have 15+ years of experience with web and it is not of "doing table layouts all over again" kind.
My point is: with enough experience, practice and interest it is possible to full stack expert without quotation marks, and more. You may not reach 98% of "expertness" needed for some one-of-the kind project, but 85% will be more than enough for the rest of the project we deal on the daily basis.
Simlar to natural languages, when the more you know the easier it gets to pick up another because you start to see patterns and differences the more of related tech you know the better you get at understanding other.
You will never have enough time to be really good on every part... except you have no private life.
I can develop "everything" too, from Assembler, many Basic dialects, VisualBasic, C, C++, PHP, JavaScript, CakePHP, Typo3, Node, Java, Android, MySQL, MongoDB, Apache, Nginx, to different CloudServies (S3, Rackspace) and many things i forgot in my 25 years programmers life. I can use them all, but i not assume that i can use them all like somebody who is specialised and thats the difference.
Yes, full-stack developer are needed, because they have an overview of many things, but in detail a specialised programmer is meaningful.
And your are right, 85% is for most customers more than enough, but the really good payed jobs are not from 85% of this customers ;-)
