As usual, this is written from the perspective of a novice/amateur table tennis player, not a professional athlete or a veteran coach, so your experience may vary. This is my experience, and my opinion.
When I first got involved in the sport, I was repeatedly told, by various sources of varying levels of skill, that I should emphasize primarily speed (controlled amounts of it) and not spin. One notable example was that forehand-to-forehand counterhits should not contain any spin. For a good while I took this blindly to heart and tried my damnedest best to not ‘brush’ the ball when counterhitting, while attempting to maintain accuracy and tempo/speed. Never quite worked. Backhand counterhits, on the other hand, were ultra-consistent when hitting with another player with similar consistency and accuracy. The reason for this? I’m not too sure, but behind the ball and the racket probably has something to do with it. And spin.
Somewhere along the line between my beginner days and today, I finally grasped a few things, one of them being that spin can aid your development. It aided mine. While my backhand was, by and far, a self-enhanced stroke that did not get a lot of external input, it has become a consistent and stable (if not quite powerful) stroke of mine. My forehand, however, was a tale of stark contrasts. On moderate and higher speed strokes (note: strokes not shots), my forehand strokes almost all contain a moderate to heavy element of spin, so they are not quite as fast as the arm swing would suggest. The moderate speed strokes are by far the most consistent of the lot, with the fast strokes a somewhat middling second, but oddly enough, it is the slow strokes that fall terribly far off the chart in terms of accuracy and consistency. This doesn’t happen with my backhand, which follows a somewhat linear pattern of speed and accuracy while maintaining an acceptable level of consistency.
I figured out that my forehand grip, stroke, body mechanics, are all brushing-oriented. No, not to the extent that I cannot flat hit if I have to, but without giving much thought, my shots tend to have an element of topspin to them. Not quite enough to hassle a practice partner unless I intend it to, but it is there, and markedly different from dead no-spin. The very fact that the nature of a forward-moving ball hitting a stationary surface generates slight topspin is enough justification for me to say: learn how to control and USE the topspin, not fear it and shy away from it.
In the table tennis world, spin is the single biggest control factor of the ball’s flight path after it has left the racket. It curves down, it rises up, it veers left or right, and sometimes it just rolls sideways. All achieve the same thing: it makes the ball path predictable and controllable (at least for the originating player). Topspin brings the ball down on the other side and, in most cases, aids the other player in getting the ball OVER the net and back to the originator.
So why shouldn’t it be used in normal counterhitting? No reason why. In fact, I say it should be. True, the notion of not generating significant topspin is the point of the counterhitting, but all involved should realize that unless you intentionally stroke to eliminate the slight topspin generated, you will end up facing a minor amount of topspin anyway, so just live with it and learn how to control the damn thing. If you’re of a decent enough level you’ll know how to control the amount commensurate to you and your partner/opponent’s level. If there’s too much spin, your opponent is either using a stroke that’s meant to generate topspin or you’re just not doing it right (wrong body position/contact point/distance from table etc).
Above all else, I think that instead of trying to isolate just ONE aspect (Speed/spin/contact point/ball arc and height etc) all the time, one should in fact tailor your stroke and shot to the emphasis at hand. Note that an emphasis on one aspect does not mean isolating that one aspect to the neglect of all else. Rather, it is the conscious concentration that separates emphasized aspects from other maintained aspects. Isolated aspects simply mean you don’t make a connection with that aspect to any other, which in most cases simply harms player development.
Found all that out the hard way ’cause no professional coach was on hand to teach. I sure hope my increasing awareness is indicative of my approach and development as a player.