So what? Yammy apper at the same time as Ulq, but that don't make him irrelevant. Uryu was introduced after Urahara, Keigo but he is "mainer" that both of them.Cause he was the first that appeared, he was the one that most threated with Ichigo, the one to get Orihime, to mess with her mind, to do almost everything against Ichigo and yes he was the strongest because of Segunda Etapa
Grimm rivality with Ichigo was potrayed from start of Arrancar arc, to me it looks like bigger than Ulq's doing.
Ulq is stronger than Grimm, but not the strongest espada.