If you post articles without the tracking code to twitter, nothing happens. So it is not necessary as people will be able to see your page.
However, if you have a tracking code and someone signs up to hubpages, you get a referral and also, with a tracking code, you can keep track of where your traffic is coming from.
You should try to use the code all the time if you care about referrals and also, if you want to track where the traffic is coming from.
You can add a maximum of 20 different tracking codes.