I have a handful of square poles. Cons are that they're heavy to carry around. Pros are that they're stable on the ground (meaning they don't roll if stepped on) and they're nice and heavy when building jumps. I will strategically place them as front or back rails of oxers on occasion if I have something I'm working on and am worried that a horse might get careless and roll a pole.
The other pro is that it's a lot easier to pick up a 4x4 from Home Depot and paint it than it is to pick up the 4x4, strip it to an octagon and then paint it (which is how we did all of our other poles).
GM used square poles in a ground work exercise during the clinic i audited last year. I'd never used them and was nervous looking at them (because I kept seeing a fetlock getting scraped in my mind, and also because im a total weenie anyway) but the horses didn't even blink at them. Not surprisingly ;-)
I haven't seen them used as jumps yet though.
My trainer has some that she breaks out if the horses aren't picking up their feet. Hitting the sharp edge gets their attention! They don't seem to be any harder to knock down than round poles, so it isn't a safety issue in that way.
I have used square poles for years. My jump cups are the plastic ones that don't seem as deep as the old metal ones, so they do get knocked down if hit hard enough.
I use them as cavaletti. If the pony knocks them, they kind of drag, but don't roll.
They are heavy, but IMO any wooden pole is!
I like a mix of poles, but it is hard to find the round ones without doubling their price with added shipping.
Proud to have two Takaupa Gold line POAs!
Takaupas Top Gold
Gifts Black Gold Knight
IMHO the risk of using a square rail as the back pole of an oxer is not worth it. Especially when either rider or horse is green. Although it is seen frequently in at home schooling set ups around here, that doesn't make it smart. Its a red flag to me and a sign of a sketchy trainer.