So in fact we've had several questions.
One is, why underrotations are called at all. Well, then, why would a Quadruple Lutz be worth more than a Single Lutz. Rotation matters or it doesn't.
Then, another question is: why underrotations are called that much. This I think would be a question, and I suppose that it was behind the establishment of the q, but mathematically the q doesn't exist, theoretically 90.001° missing is still an underrotation with 0.8 Base Value and calls, while 89.999° is a mere incomplete rotation. (In practical calls, it seems to depend on who's jumping, but that's quite another matter.) A real q category would name an interval, for instance between 85 and 95° missing. Overall, I agree with Lariko that a well hidden slight underrotation or q shouldn't be penalised beyond the Base Value adjustment, or a more severe GOE call, rather than both. I believe that this aesthetical dissimulation of slight underrotations could be measured automatically in time, but not soon, unlike the rotation itself.
In fact, in the few artistic disciplines I watch, an occasional slip isn't punished much or at all per se, as long as it's well managed so as not to interrupt the flow of the piece. I don't know if you've ever seen a fall by Sylvie Guillem or Svetlana Sakharova, but an untrained eye may not notice it. A fall! Managing falls is a full training in Ballet. (A caveat, is that these Ballerinas are not in a competition as we call it here. The only Ballet competition looking somehow like a Figure Skating big championship, the Prix de Lausanne, would be like a Junior World Championship with younger and older Juniors separated, and with a very significant part of the competition partly streamed only, where the main objective wouldn't be which competitor has the best performed program now, but which are better hearing and understanding and attending to coaching advices by the official coaches of the competition; where some of the best candidates wouldn't participate because they're already enrolled in full training in the training teams the candidates dream to enter; and where the global objective would be for schools to recruit the best Senior Students, rather than to assess the best dancers at the moment.)
Of course managing falls is a full training in Figure Skating too, and from the start; and Novice rules have special calls depending on how long the skater remains down, but there's no incentive on managing them aesthetically (I believe that in the IJS, it ought to be a slight plus in all three Components, depending on how it's managed of course) and I would say, with what I've seen of scoring, I've seen no correlation at all between how a Figure Skater manages a fall, and their Components; to the contrary, the so-called cap (which, being supposed to apply also to skaters not awarded high 9s or 10s, isn't a cap but a deduction in disguise) for "serious error" was a step in the wrong way (all the more when some skaters could fall and not get it, to the point that Lakernik called it facultative; while some others would get a cap for slighter — or invented — errors; always the same skaters) because it was forbidding to consider the skater's skill in managing/concealing the error so as not to impact the aesthetics of the performance. Consequently, while most skaters work on raising again effectively so as to miss as little distance and steps as possible, the aesthetic impact is more rarely worked on. This is, in my opinion, a point where Figure Skating might fail to be artistic enough (it would still be rather technical!)
Nevertheless, a recurrent error in Ballet or Music competitions would be analysed as a flaw and treated more harshly; for instance, the same error in the same jump at both Short and Free Skating, or systematic underrotations. It's not just because it's a sports where athletic prowess is measured, it's also because it is artistic, although there have been immense artists with technical flaws (Anna Pavlova!)
In all artistic disciplines, I believe that technical perfection (or closeness to perfection) is an essential element. Can I take two recent First Dancers at the Mariinsky as an example?
One had a faulty technique and looked blurry even to the untrained eye in spite of his charisma. He got important prizes but the public has always wondered (to be polite) why he had been appointed a First Dancer, for a start; or even a Coryphee.
The other one had been hailed from school for her gorgeous, faultless technique and was announced to be the future Diana Vishneva and recruited by the Mariinsky at the end of her penultimate school year, which is rare. During her last year of school, few noticed that she was already showing signs of rebellion against the minute directions of her mild, patient teacher, and the school team managed to hide it in the graduation show. Once full-time in the Mariinsky, with new coaches, she was promoted to First Dancer in weeks, something never seen, but seemed to be as rebellious to her new coaches. She was still luring many and won prestigious awards. Yet when you dance the part of one of three Bayadere shades you must dance it perfectly in unison with the other two, not throwing your leg higher than their 90° (as part of the Choreography) because, you know, a leg must always point to the Sky... She's still a First Dancer, doing first parts, with a waste of a glorious technique, but still more watchable than the first example thanks to this very technique.
Of course, we're speaking of the whole meaning of technique, also in Figure Skating, not just the types of jumps. Skating Skills; precise, explosive, purposeful, harmonious movements of the whole body, and so on.
so I try to avoid using it myself. Of course, no one is perfect. 
It's on the skater to perform so well that the judges will not be undecided about whether their elements are pristine or cheated.
