I just barely started setting up a bioactive tank myself. It'll be for a carpet python I am thinking in the future. Right now I am housing a BP in it though. I added a drainage layer for a couple of reasons. First, I happened to have some clay hydro balls on hand. Petsmart was selling them for like a buck or two for a big bag so I bought all they had at the time lol. I knew they would be useful one day!
Second, I want to keep moss growing in my tank. So I plan on spraying down real good daily. I even picked up a one gallon sprayer, the kind you use for yard treatments, so I can spray it much easier. The drainage layer for me is just a little insurance that if I happen to water too much it has a place to go instead of making my tank into a swamp. I planted a couple spider plants I had laying around and the bigger one I actually planted all the way into that drainage layer so if I get any water down there the plant will help to absorb it up. It does pretty well in standing water, I've tried before with it.
The third reason is that by using about an inch of clay balls, if standing water ever does get to be an issue I can always scooch the substrate over and siphon out some water fairly easy.
I used coconut fiber, husk, peat moss and vermiculite in my bioactive tank. All absorb water really well and it also gives me fast drainage too. I did about a one inch drainage with 3-4 inches, depending on the part in the tank, of soilless mix. I think it strongly depends on what you plan to keep in the tank and use as substrate that will determine if you need a drainage layer. Mine is more for peace of mind than anything honestly and because I happened to have the supplies I needed on hand.
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