No you don't. First hand knowledge with having Flourite for 15 years. I've got several corys that are perfectly happy with very long barbels.
As far as why you may need additional stuff in your substrate - well, look at terrestrial plants. Do they grow in fairly clean soil or fairly messy stuff? It's pretty messy and filled with organics and other nutrients.
Flourite and others help add some of what plants need. Dose the water column + detritus and you end up having what the plants do need. If you start CO2 dosing and pushing up your light, you might need to do more, especially for heavy root feeders.
Root tabs for large swords, crypts, or other heavy root feeders is never a bad idea unless you plan to moving them around. Then it just gets the water column messy.
I've used Jobe's plant spikes for years w/ no ill effects. Even when they get pulled up - just be particular about a WC and sucking up the extra bits.
florite is cracked clay and is very sharp you should put sand or mix it with some gravel if you have barbel fish going over it, like corys
shows you whats in it. It has def helped with my plants in general. I dont use tabs i just use liquid/powder foods