It's not necessary, especially if you have a good case with a raised lip around the front.
I sold my 4S for 350 just a few days ago and I had had it for two years with a good case but the screen was left untouched. It had been dropped quite a few times, including twice from above waist height on concrete and the screen didn't have a scratch on it.
I mean, it definitely does help, and it really does provide piece of mind I'm sure. Like someone else said it's definitely just a personal preference.
I just prefer the feel of the glass screen without any barriers in between.