In a recent question about comparatives, a dispute arose in the comments about gradable antonyms like useful/useless where English speakers strongly prefer to use comparative forms only for half of the pair.
I proposed that this is simply an example of marked word pairs, a kind of asymmetry where one word of a pair is the dominant or default form for general use. Marking explains why we normally ask, “How old are you?” instead of how young. I think it also explains why it sounds awkward to say, “This is more useless than that,” unless we add emphasis: “This is even more useless than that.”
Edwin Ashworth disagrees, claming: “Useful is not an absolute adjective, whereas useless is. The opposite of useless is possibly essential.” I take this to mean that useless properly has a complementary antonym like on/off or day/night, where the two meanings don't lie on a spectrum and don't normally permit comparatives. But essential and useless still form a gradable spectrum, and phrases like more essential are not at all uncommon. I doubt that it's even possible to find a complementary antonym for useless, as utility is inherently gradable.
However, another possibility occurred to me: Is there a class of half-graded antonyms, where the two meanings lie on a spectrum, but only one of the words is gradable, while the other is absolute? This would work like a dimmer switch, where instead of on/off, you have something more like bright/off. One such antonym might be doubtful/doubtless: Wiktionary lists the adjective form of doubtless as not comparable.
My intuition is that adjectives like useless and doubtless are simply more marked than most, such that we permit comparative forms only in very limited contexts. However, I'm open to the possibility that some gradable antonyms really are anchored at one end by an absolute, uncomparable adjective. Does one of these theories have more merit than the other? Or are we just saying tomayto, tomahto?
Update: Thanks to ruakh for helping my organize my thoughts on this question. Essentially, I noticed that there are three kinds of adjectives that are similar in form and meaning but with different limitations in common usage. Some (doubtless) are listed in dictionaries as having no comparative form at all. Others (useless) have legitimate comparative forms, but they sound awkward without other modifiers (a hammer is more useless than a screwdriver). Still others (thoughtless) sound entirely natural in comparative form (Bob is more thoughtless than Alice).
Because all of these examples naturally fall on a spectrum and share the same suffix, I suspect that these differences are a matter of convention rather than semantics, but I'm not sure. Ideally, I'd like to see an analysis of the non-comparable and less-comparable words, specifically: How common are they, how strongly do people avoid the comparative forms in practice, and what role does convention versus semantics play in determining their limitations?