While there's nothing wrong (in my opinion) against the circular form (my avatar is a round character on top of a ball), circle profile pictures are the default on this site. Not using custom profile pictures confuses users and degrades people's sense of individuality. It also helps people get away with abuse because flaggers don't have a unique avatar to tie repeated abuse to.
There are way too many circle profile pictures and it's getting ridiculous! The circles have overthrown the long-standing square government and are planning world domination. We must stop them before this round enemy takes over. We will reinstate the quadrilateral government into power and achieve peace, this is a necessity! Do you think I wrote all that just to reach the minimum character amount? Yes, yes I did.
This website obviously needs more circles! Yes! This site needs to exhibit what nature fails to comply with: the perfect circle! This website needs more circles, specifically more pictures that portray the existence of the perfect circle because we delicate and fragile lumps of muscle and bone need perfection, and the first step towards this perfection is through none other than the circle. May circle profile pictures live long and prosper!
Who cares what someone's profile picture is? Does the circle by any other name fail as hard at reasoning skills? Name, face, whatever? I judge someone by the rational of their argument, their ability to use grammar, and how annoying I find their complete lack of ability to cite sources. If I start to notice a trend in circle avatars being horrible spellers, I'll let you know and we can carry the flag of self-righteous grammar Nazism of into the sunset together. Until then, "stay thirsty my friend".