I did GMail-myself backups before Dropbox was around, and now have stuff on a Time Machine backup as well. But the networked backups are dependent on Google and Dropbox actively maintaining their servers for me, which it makes sense for them to do, but it's hard to depend on. If Google lost all traces of me today, that would be fine because I have a local backup, but it would be no big deal to them because I'm not a paying customer. As far as I know, they don't even have a legal obligation to maintain my data. If I have a photo or text I want to be available in 30 years, I think that printing it out and sticking on a bookshelf is much more reliable than depending on a nebulous third party or trusting that I won't inadvertently lose a set of backups.