I know some people do it because they find the crescent one in 4-1* Which is a stupid reason because you could make a better crescent with just a few shards* But most of the people I see don't use a crescent version so why do people use it when the kilij is clearly better?
i suppose the main reason is because of the 4-1 C falchion.. they get it and use all their darkmoonstones on it. then they find the kilij, check the stats, and they dont look as good as their newly upgraded +4 C falchion, and assume it's not as good.
but i'm betting only inexperienced players make that mistake. people who take the time to study the weapons know to use the kilij.
I still use a Crescent Falchion+5 on my 130 Mage. I've considered upgrading a Kilij but look at the stats. CF=112/119 CK=119/126 both with an A in magic, I'd rather not go through the trouble for 7/7 more damage.