A supplement plan is not necessary, but not a good idea to consider.
When you are on Medicare to pay about 80 % of your medical expenses (doctor visits , hospitalizations, ER etc ) so if you were to go to the hospital without any additional insurance then you can easily go to $ 3000 on invoices. That's why supplements are available. However, depending on what type of coverage you want, you can end up spending $ 200 or more per month on it.
Therefore, the privatization of Gov't Medicare and Medicare Advantage plans created as an option for all Medicare beneficiaries. These plans will not cost more but offer more protection than a regular Medicare .
This is a good article explaining the differences between the two.
http://www.helium.com/items/289436-medic