The reasoning behind this, I'm not 100%, but I can give you what I feel is a fairly good speculation...
When I set up my first tank, I had absolutely wonderful water parameters for a few months immediately following the initial cycle. My water and lights could have supported a sebae easily. However the following month I got a crap load of cyanobacteria. My water parameters were good enough for fish, but had I bought a sebae when the parameters were excellent, it would have been dead at this time in the tank's life. Some people don't realize it, but the tank's cycle is far more than just the stability caused by nitrogenous bacterium. By 6 months, you can usually guarantee that the tank is stable.
One last thing. Had I reread my first post on here, I would not have pressed submit reply. It came off the wrong way. I'm glad you forgave me and we can move on
I agree 100% with Meowzer, it is almost impossible to wait too long in this hobby.