No. The built-in hub in the Echo Plus is NOT necessary.
If you already have Phillps Hue lights, the Hue Bridge is all you need to control your lights with Echo/Echo Dot/Echo Show. And neither the WeMo Switch, nor the TP-Link Switch require a hub at all. So, no you don’t need an Echo Plus. An Echo or a Dot will suffice to control lights, smart switches. I have more than a dozen Hue lights, a WEMo Switch and a TP-Link Switch and they all work fine with my Echos and Echo Dots.