I'm not saying substrate heating won't benefit...but it's certainly not necessary.
I believe it has more to do with creating convection/heating currents in the substrate, which improves waterflow around the roots, so they get better nutrient uptake.
I think they're just more hassel, and are probably more beneficial in the deep substrate tanks which are layered. Otherwise you end up with plant roots tangled around the cables, and then you remove plants and up come cables. Too much hassle for the minor benefit.
Former advisor and planted tank geek...life's moved on though.