I'd be able to fit more square

Exactly! And that is a BIG advantage IMO.

I do hundreds of plants for sale and won't use round pots unless I absolutely have to for some reason. Garden Centers sell for the small home gardener so somewhere along the way they decided that no one cared. Plus the round ones are usually cheaper to order so they can mark them up for more profit.

Professional suppliers sell far more square than round.

Dave

None that I know of. I don't find square at garden centers often either (although I can sometimes at better nurseries that carry supplies). I will order in bulk every few years....much more efficient use of space both indoors and out.

I prefer square slightly taller than wide, my own are 3x4 and 4x5 - my most used size for 'growing on' perennials and shrubs. Last I purchased are ribbed to encourage downward root growth, fewer roots encircling the inside of the pot.

I prefer square also. Space-wise it seems more efficient especially for small plants. The only advantages I've noticed with round pots is that once the young plants get a bit bigger it gives them a bit more room also some plants tend to grow wide and when moving them around some of their leaves can get crushed by the neighboring pot.