If you're going to Torrent you need to keep one in your pocket at a minimum.

Public or private,you should be running hiding your ass through a VPN or seed box.

Private trackers run ratios to make sure that content stays available. Well you can find most of what you want on public trackers there are always a few things here and there that are much easier to find in private.

Security-wise I don't really think there's much of a difference. Private trackers get infiltrated and shut down they're just much smaller when it happens so you don't hear about as much.