I recently resumed the use of charcoal in my filter and wanted to add that it will absorb excess chemicals and dissolved organic matter that plants cannot or do not. And because activated charcoal is so fractured, it has enormous surface area for nitrifying bacteria to populate. Finally, it helps keep the water smelling very fresh (the result of removing the aforesaid chemicals/dissolved organics).
If you change it regularly, leach-back should not be an issue. And if the tank is heavily planted, i.e., a jungle, then charcoal's benefits will not be as apparent. But for moderately planted tanks, it is useful.
As a substrate, however, I have no opinion.
Tank: 40 Gallon Breeder w/Eco Complete ||
Aquasun 156W Quad 5000K || SunSun 302 Canister Filter (Sunsun Pimp #77) || Pressurized CO2 || Dual Current Gamma 15W UV Sterilizers || Hydor Inline Heater