the show wouldn't be as good then. they help build intense drama to upgrade the episode plots, like in "Camping Can Be Cool" and "Skunked". they serve as characters who prove how Mordecai and Rigby would really act to girls. also i think Eileen is so cute.
The plot of some of the episodes will be different, so from this statement, I'm assuming it would be a bad thing, since Mordecai is in love with Margaret and Eileen...likes Rigby and causes awkward moments in the show.
Basically I really like Eileen..Margaret is good too..and they make great additions to the show..but If they were taken out Mordecai most likely wouldn't have a love interest..well if they both just went on a "vacation"..or something like that and would be back on the show after a few episodes I wouldn't mind but forever..I don't think I would be fond of that..but still at least it's still Regular Show.