The couple (their names are Vanessa and Beth) have quite their justification: Vanessa as a platonic friend for Frank to talk to and now also talk about the ways of Becca, Abby and Lilli and who doesn't want to get into his pants. That being said, it can't hurt for Frank to have friends of influence, depending on when (and it will definitely come to that with Shannon at some point) it goes nuclear. That being said, it was definitely interesting to see for once that there are people (Vanessa) who weren't impressed with Becca's whole act.
As for Meiko and Abby: I think because she's been rather subtle so far (at least more subtle than Becca and Lilly), she's underestimated: Abby is definitely not nearly as harmless as she seems. In fact, with the rules she sets, she pushes Frank further and further in the direction she wants. Becca once described Abby as more of the submissive type (and herself as more of a mix of dominant and submissive), but over the course of the plot we've seen more and more of a shift, at least in Abby and Becca: Becca is, and we see this clearly in her dynamic with Meiko and in the confrontation with Vanessa and Beth, she's definitely a purely dominant type by now. With Abby, I say that she is now a mixed type, because she is clearly more subtle than Becca, but tries to get her way with her rules towards Frank and in the confrontation with Lilly, when she managed to get Lilly to become meek, something that only Shannon has managed in this way so far. We don't know yet what exactly she told Lilly, but she gave a rough summary, and I think it takes little imagination to guess the approximate wording.
In general, Meiko is more the innocent person for me, Abby has at least experimented with Lilly and "Big Daddy" in the past. It's different than with Frank, but still you shouldn't underestimate Abby, while Meiko has no experience at all.
Outside of that, Meiko has another role: Frank has never been Meiko's stepfather or foster father, unlike with Becca, Lilly, and Abby, and so can safely act outwardly as Frank's girlfriend without the risk of Frank coming off as a groomer afterwards.
However, I agree so far that no more girls should be added. Since we've seen Madison (the girl with the tattoo) again we can assume that we'll see her every now and then, the judge's daughter (and probably the waitress we've seen twice so far) will probably remain one-off stories. With Madison, I personally don't know where to put her, since in fact pretty much all the roles are covered by the other ladies. Also, I hope Vanessa continues to be portrayed as purely lesbian and doesn't eventually get curious for some reason about what the hype is about Frank's wand. That would ruin her as a character for me personally.
