You have to sharpen the saw somehow and by taking an existing site you can focus entirely on the interface and not on requirements capturing or brand establishment since those have been provided for you.
It makes us designers stronger. One of the first things I tell people trying to better their design is just that… redo your favourite website. Learn the techniques they used, find out what they did what they did and then mess it up and make it better.
In fact, in one way, I made this so I could spend more time on my startup. I found myself wasting too much time reading Hacker News. Either I was procrastinating and refreshing too often, where I found it hard to either identify what story was new that I hadn't seen, or which ones I'd already seen but did not interest me. Or, I was going back and reading simply so I wouldn't miss anything, since stories tend to trail off pretty quickly unless they get a lot of votes.