What I don't understand is why Frontier doesn't pick, say, 2 destinations from ILG and fly it daily (OK, if they want to do 5x or 6x that might work).

It's just not practical to fly 3x a week. You're going to get horrific yields, because you're basically offering nothing except price at that point. People who would be inclined to be their customers go elsewhere because, for example, you can get them out on Friday.

Given how high fares are at PHL right now, Frontier would have a shot if they offered plausible service from ILG to a city that people want to travel to. I mean, look at what Spirit is doing: their service is horrible, but they get people on price (and then charge them for lots of incidentals).

If Frontier offered a reservation and in-flight service A LITTLE BETTER than Spirit, with reasonable frequencies, they might be able to build a business out of ILG. And one advantage of ILG is that US might leave them alone. For instance, in response to Spirit's PHL-DFW service, US is charging as little as $44 each way on that route. There's no way Spirit can make money if US is willing to do that.