"You have 2000 members but only 200 of them actually upload a user picture.... now your site is full of "default" user picture"
Imagine if you have that kind of problem lying in your site. Sure it wont break your site. Just makes it a bit "ugly", is it acceptable?
We all know that user are lazy in general, asking them to click and upload a file is a daunting task for them. They will just either ignore it or flip away from your registration page.