Its also about consistency. While after a few months SOME peoples tanks are stable, the hobbiest themselves are still unfamiliar with the tank. Water changes, salinity changes, mixing the right amount of water, adding water, taking water out, changing filter pads, all of these things can contribute to an unstable tank. So even though your tank may have zero's accross the board and your temp, light, and sg are right on, that doesn't mean that after your water change the following week it still would be. I found it took me about 5 months of just water changes before i felt completely comfortable with the process. And it took me that long to get the mixture down pat, sometimes i would add too much salt to a new batch, sometimes too little, things like this can contribute to an unstable tank as well. So it is best advised to wait until your tank is stable and you are totally familiar / comfortable with the general maintenance of your tank before adding something as fragile as an anemone.
Your tank your choice, we are just advising you not to do it based off of the success rates with new tanks. Like john said, it doesn't mean it won't work, but I for one would be advising someone to purchase one because it worked for me, we need to look at the larger picture and realize statistically the odds are against you if you purchase one.
