You will certainly learn a lot in the process, and be more useful to future companies if you know not only how to use an engine, but how game engines work and how to solve problems other API-level programmers will struggle to fix.
I've developed several game engines myself, and each time I do it, I find better ways to do things along the way. Of course, building an engine isn't a weekend hobby. Getting a basic rendering core going is something that can be done in a few months - but really making a fully-featured engine can take years.
Edited by Jeff.Leigh, 31 May 2012 - 05:27 PM.