Greatly exaggerated in my opinion.
Substrates don't usually get anaerobic unless they are very thick or packed. So heating it won't really improve that aspect much.
Aquatic plants also absorb nutrients through all exposed surfaces - this means through their leaves and stems as well as roots. So I doubt that the minuscule amount of current generated by the temperature difference will really improve nutrient delivery to plants.
Most of the time getting enough CO2 to the plants is the biggest lynch pin in growing plants.
As far as I have read most people have not seen a noticeable increase in plant growth with an under gravel heater. That isn't to say that it won't help, but the difference is so small that it isn't measurable.
I also considered getting an under gravel heater when I saw my LFS selling one for a discount price (40$), but then I spent the money on more lights for a new tank instead
