When draining your pool, do not allow the water to go anywhere near your septic system. Otherwise it will "flood out" the system.
Michael Foster http://www.knoxpumpingservice.com/en/services.html
This tank is designed to overflow into the leech lines; this is how it maintains it's level. The liquid overflows out as the bio-chemical process is taking place. You should be adding either RIDEX or common YEAST (cheaper) every year to help the process take place.
Emptying out the tank is removing the solid/heavy stuff that will accumulate in the bottom. Some of this. over the course of time, is breaking down.
It would be a waste of money to empty it too often.
When you see the ground above saturated and stinky; that means that the whole system is full and the ground cannot absorb any more.
When are you draining the pool water; surely not into the Septic tank or the ground around the tank? That would be utterly ridiculous!
From what I've seen, every 4-5 years is sufficient for a small family; but you should get other opinions. It would depend upon how much water you put on the ground around the leech lines; rain and sprinkling, so that it doesn't accumulate in the leech lines.
Good luck and may God bless