Having this as default and not allowing the user to change it on the login screen reduces the barrier to having to sign in when returning to a site.
This can be off putting depending on how frequently the user visits. Depending on the tech level of the user and whether or not they can remember their password may impact whether they even make it past the login screen.
If they're always logged in when they browse, even if they're a lurker, then there's a much greater chance they'll post as the reply box will always be there for them.
I bumped into a member of one of my forums on a facebook group and when I linked them to something on the forum they had to reset their password and jump through hoops to login because it had been so long since they visited. Keeping them logged in will help minimise issues like this.
Many of the big social media/community sites no longer display a checkbox:
- Google Plus