Well, Renji (in Episode 30 & 31) did say that shinigami spirit levels were capped when in the mortal world. So perhaps that is the reason they wouldn't necessarily be able to take on the Menos alone on the mortal plane. Let's also not forget that Ichigo received a little aid from Ishida to defeat the Menos (which the Menos actually looked, to me, like it just decided against venturing further and just retreated).

-t