I'm not sure why this matters so much, in the first episode they already showed a shift in the meta from AGI builds to STR builds so all the old top elite players got outdated and ended up more or less useless. It just means most find the light saber to be a toy and therefore find it useless, to be fair most probably don't have the reaction needed to do something like Kirito do with predicting the prediction line. It's really first when people break the mold and try things out that you actually find out it's worth and it just seems that most people haven't yet so I wouldn't believe everything she says.
So I guess that leaves us with the question, why hasn't anyone else done it yet? Besides the reaction needed to be as proficient as Kirito I'd say the most plausible explanation is time. It's been less than a year since Kirito came back from SAO (Nurse ep3), he'd probably have needed a month (complete and utter guesswork) or something of rehab before they let him out of the hospital + some moping around waiting for Asuna to wake up and the whole ALO thing. So the seed has probably only been on the net for around 10 months, GGO has been around long enough to have pro players that earn money playing and on their second or third major tournament. For the sake of it let's say they have one each month so it's a game that's been around for around 3-4 months? Been in development for around 6-7 months? Yup those pro player must absolutely know everything there is to know about every weapon and how useful they are. Bad writing from the author? I'm inclined to think so. But yes this is the biggest reason why I think nobody really knows how useful light sabers are because the game has most likely not been out for long enough.