Cost wise it isn't worth just having one route...unless...it's one of your star performers.
Oh come now. That's really not the case in the industry at all. Is Manchester now the only airport in the whole of Ryanair that is served as a spoke from a base? If so that's a shame. I know easyJet are happy to serve single route airports.
They did release their top ten routes a while ago and Manchester - Dublin was definitely in there.
Was it by volume or profitability?