Do you NEED one? The short answer is no. The long answer is maybe. At my seasonal site when everyone is there on a weekend and running their A/C in 90* temps and the low voltage bug bites I am ok with it. This winter while in TX I was shut down twice by high voltage. Now my unit is an PI EMS and has more than surge protection. Why did I get the EMS? Three years ago when in TX a thunderstorm went through and I only had the PI surge protector (30 amp). It took the hit instead of my RV. When I went with a 50 amp RV I bought the EMS.
It is up to you how much burden you are willing to bear in the event there is an electrical problem and you suffer loss due to it.