Unfortunately I didn't spend enough on my grinder to get one that would do square chisel. In the past I have always hand filed my chains, and If I'm in the woods and happen to dull a chain, it's nice to grab a file and fix it on the spot quickly. It just doesn't seem like the sqaure chisel would be so easy. I have toyed with the idea of learning the square chisel, but I'd probably start by buying some square ground to use for a reference, and then try some filing. I just think that the average user has a hard enough time keeping round ground chisel sharp, and they would get more benefit from that than going to square chisel. Some people I would recommend chipper chain to, as I know it is going in the dirt on the first cut!
If you look at productivity over a long term, seems like the gain in using square ground versus round ground would likely be more than offset by the extra time required to maintain the square ground chains. Looking at a few cuts in ideal conditions where you want to feel the saw slicing like a hot knife through butter, sure, I'd love the square ground.