Well, in all reality, you don't "need" anything. It's really a matter of how comfortable you want to feel protected, like insurance.
Think of it this way, you probably paid $50,000 or more for your new 5er. $300 is nothing compared to the loss and expense you'd have to replace out of your pocket if something happened that overloaded and blue your electrical up and all your appliances.
Keep the 30 amp protector and use it when you don't have a 50 amp receptacle. It's good protection. I've used mine on my 50 amp 5er a few times. Peace of mind is priceless.