Not necessary. It really depends on the rest of the setup. If you are going to have a hard time venting the heat, then a cooltube will help. Otherwise, you don't need one. Good luck.
It's necessary if you cannot keep temps under 85F any other way. But it's not necessary for the bulb or anything like that, it's just a way to whisk the heat away from the bulb and keep it from entering the grow space.