I started my career in front-end world with similar ideas, and copied couple of websites (mostly markup and css). So, while I can agree that it is a good thing to put into your portfolio in the beginning, when you are closer to some middle level, I can't really recommend it.

What does it show, when you just have copied websites in your portfolio? And what is the problem to _almost_ copy (e.g. create a twitter clone, but allow to use only images, or some other limitation)?

So I believe that one should be inspired by existing solutions, but don't copy it blindly. Don't know nice colour scheme? Just go to bootstrap and copy their colours; or simply get it from a website, which design you like.