A hub challenge will keep you focused on your goal, and make you somewhat accountable, which is important to have especially since we don't have bosses to make sure that we do what we plan to do. A challenge will also help you fine tune your skills faster, since you have to learn to make quick decisions and get lots of practice.
On the con side, it is tiring, and sometimes you may write a hub simply to meet the number requirement without making sure it is a quality hub.
I say the benefits outweigh the negatives. Take the challenge, maybe make up a number that is more reasonable for your schedule. If you get to a point where you simply can't write a quality article, let it go. Don't publish just for the sake of publishing. You can either make it up later or not. Doesn't matter. Either way, you'll have more written than you would have otherwise.
By the way, I've written a hub that lists the pros and cons. I wrote it shortly after I completed a challenge or two.