I think the issue of 'groundswatting' probably originated out of concerns for safety. When more than one hunter and/or dogs are involved, shooting horizontally can be dangerous. You never hear the expression 'tree swatting' - probably because it isn't a particularly dangerous practice.
My opinion is that, as long as a particular shot is both safe and legal, its okay. Ever since I got a dog, I can't remember seeing a grouse on the ground. The dog'll either flush 'em or tree 'em. I get much more satisfaction out of a wingshot, but I'll take what is presented to me.
I'll never knock someone for shooting a bird on the ground, but I understand why wingshooting purists limit themselves to birds in the air. Hunting requires skills - skill at locating your game, skill at shooting, etc. After achieving one skill level, we often challenge ourselves by adding another degree of difficulty. For my hunting partner and I, our thing is to see how many different shotguns we can take birds with in one season. Differences between patterns, location and operation of safeties, numbers of triggers, and other variables go a long way towards improving the odds for the birds.
If sheer numbers were an issue, we would abandon any such 'games'. But, getting our limit isn't really an issue, so now we focus on milking as much fun as we can while getting there.
SS