Jestar has used genjustsu before in the past against a certain Rinnegan user; it was mostly a distraction, but even with the sharingan, you do not automatically escape the genjutsu. There is a process for that, a process which may eat up one your move turns.
Genjutsu also has usage when it is deployed beyond the target's senses to reverse it. Rita's singing technique for example, or the senjutsu genjutsu deployed by the toads. They are sound, I get that, but they are examples of genjutsu that have their effects before an appropriate reaction is deployed, or are constantly ongoing.
Using genjutsu in a 1v1 fight is incredibly difficult, as it is hard to capitalize on the moment or two in which the opponent is releasing or trying to figure out the genjutsu. As in the series, it takes some serious intelligence/tact to not just deploy genjutsu, but for it to work proplerly and do its job.
I am not saying non-genjutsu userss are not bright, but genjutsu in itself is a reserved art even in the series. Even Jiraiya could not perform genjutsu very well/ at all.