Creating an engine can certainly be educational. But I think that an engine created before you have experience in creating games, and before you have been exposed to existing engines, will largely be a throwaway exercise. It is unlikely that the engine you end up with will be designed in such a way that it is actually convenient to use for creating games.

Creating something that ends up being not extremely useful is part of the learning process. No one starting out will be able to build a great engine right off the bat, it just wont happen. You need to go through it once just to learn what all the parts are that you need and how they fit together. Then you go through it again and you have something useful, but not great. Once you build your third engine (or so) is when you actually start to have something that's worth using and maintaining, because by then you know what you're doing.

If someone wants to build engines, then they should start building an engine. Start with a basic one that meets the needs for a simple game. The make the game and start over on a new better engine using what you learned.