No you don't "need" a slider. Just like you don't "need" a seatbelt when riding in a vehicle. Until you do.
I went through this same questioning when I moved to a 5th wheel. My issue was, I can buy an auto-slider and never have to worry again or I can by a manual slider and wonder every time if I should make the effort to get out and slide the hitch.
I don't regret the auto slider one bit.
Now, everyone is different. If you are skilled at backing a 5th wheel you will probably never have to jack it much past 45 degrees to get it where you want it. If you are not skilled..... you will be approaching 90 degrees often. This can be a very good determining factor as to what kind of hitch you want.