I don't think your restatement is entirely fair -- "if people don't use a product
at all it's a bad product" would be closer.
Even that's somewhat reductionist, of course; there are a lot of good products that we use only occasionally. But there are some products which are designed to be with you most of the time: smartphones are an obvious example. Watches are another one. How about a smartwatch? I think it'd be fair to say that if you're wearing your smartwatch most of the time when you go out, it's a success for you; if you don't like wearing it -- for whatever reason, whether aesthetics or because it isn't that functional (or both) -- and mostly only trot it out to show people, it's really a failure for you. If most of the smartwatch's buyers are in the "only trot it out to show people" camp, the product is arguably a failure.
And, I think it's fair to use the same criteria for Google Glass. Are most of the people who own a Glass using it regularly, or are they only trotting it out when it'll be cool to show people? Scoble is, whether he puts it that way or not, effectively arguing the latter. Gruber is in turn saying that if that's true, it means that Glass -- even if it's a marvelous technology showcase -- isn't a good product.
(As for the contention that Gruber will argue history is relevant when it suits him and irrelevant when it doesn't, well, we all do that, and I think we'd all claim that we're doing it because in some cases history is relevant and in other cases it isn't, right? Whether Android will "windows" iOS depends on whether you think the mobile computing market will follow the same trajectory as the PC market did; if you think the concept of Glass will take off one day, maybe dramatically, but Glass itself will be something of a footnote, then the Windows Tablet comparison is apt.)