Of course gain rate doesn't fully capture "skippability" but it's the simplest analysis.
Gain rate doesn't capture "skippability" at all. It's a completely different metric. You would want to measure games where it's gained at all without taking into account the number of copies that were gained.
"Virtually never skippable" is a region populated by probably Chapel and maybe Ambassador... if anything. One step further is the land of Mountebank, Wharf, and Rebuild. One more step and you get to Witch and Sea Hag.
Three steps down the road, if you look to the right you will see the coveted Goons, on a lunch break.
In 2000-2500 games of Dominion I've played, many of them pre-Dark Ages, I've been happy skipping Chapel and Rebuild many, many times. I skip Sea Hag more often than I buy it.
Mountebank and Witch I've skipped a few times, yeah. Mountebank is a lot easier to skip because of Gardens and Counting House, but Witch not so much.
I've skipped Goons twice ever and been happy about it. Neither game had a village, one had
Library (though there's some debate on that one) and the other was
Cultist.
I've skipped Ambassador once ever and been happy about it.
To be fair, my opponent didn't buy Ambassador either. (Wow I watched this game again, I can't wait for all the people to tell me I should have gone with Black Market here...
I don't recall ever skipping Wharf or Tournament and being happy about it, but I may not have remembered these things. On a very fast Ironworks/Vineyard board I could certainly see myself skipping wharf, in fact in many rushes (IW/Gardens, Beggar/Gardens) or other combo decks (NV/Bridge, Hermit/Market Square, maaaaaybe Masterpiece/Feodum) I'd consider skipping Wharf and/or Tournament (or like all of these cards really)