The Ramps can reproduce fast, assuming they can still reproduce. Susan was a physical threat from Night One, and she hadn't even fully turned.
We don't know a lot about the Fomor, but they seem to be interested in having fodder. Hence their use of magically-enhanced servitors.
An individual Ramp is more of a threat than any of the servitors we've seen so far. So why wouldn't they want them?
First, that's a pretty big assumption. I'm having trouble accepting that something that seemed to require ritual involving physical components can happen anyway essentially by accident. The Red Court have always seemed the most "physical" of the vampire courts, and the literal, physical blood seems to have been necessary to elevate Bianca.
As for them being more powerful... eh, I'm not totally sold on that. They have a few crippling and -- most importantly -- well-known weaknesses that let pre-infection Susan take a few of them out.
Truth is we don't know what all their servitors can do. Having minions that don't have glaring weaknesses or an addiction to blood might be worth the trade-off of power.
We're talking about a pantheon that called other broken pantheons to their side, to unite under one banner. The Ramp survivors would fit that perfectly.
If there were more than the two most cowardly examples of the species, I'd agree with you more.
Think of it this way, though. Let's say they show up in Peace Talks on official business. Harry knows where they were last seen. He's on speaking terms with the guy who had them. You don't think he's going to ask? You don't think everyone else -- like the White Council contingent -- is going to notice that the Fomor had been allied with a group that had so recently tried to genocide them?
The Fomor or Nemesis would have to show a few of their holding cards to get them out and keep them out. I don't think it's worth it.