In former times I always wondered why people were willing to pay for the professional version of Flickr.com 20 US$ a year. The service was not that great and frankly you could get for less bucks quite more bang. So the answer was simple: because of the social functions there and the big community.
Nowadays the professional account has gone and you get one terabyte for free. So why should you still care if you are using Flickr or not? Quite simple: because you are giving your pictures out of hand. If your pictures are against the terms of service of Yahoo! they are going to pull them either down or behind a "adult verification" wall. This has already happened in the past with pictures of greek statues and such, making the photographers quite angry.
If you put your pictures up to Flickr you are giving them out of control. So, if you really do want to be on the safe side, at least make your own backup of them regularly or keep your own backup. Having a backup never hurts, and you can never trust a company like Yahoo! to be as passionate about backing up your data as you are. They just use some quite good, though still automated tools and they don't know you.