Yeah you're basically right, google isn't going to know if all the bookmarks it finds were made from you using the same IP. But you're going to be somewhat limited in the number of backlinks you'll be able to make.
The reason you need to use proxies is so that you can create multiple profiles and accounts on the same websites. It sounds like you understand the point of them for being able to post on sites like craigslist from multiple cities, but your wondering about the use for building backlinks.
Take your social bookmarking example for a start. Say you take a list of the 50 most popular bookmarking sites. If you just use your real ip then you're only going to be able to make a limited number of accounts on each one of them to post the bookmarks. Say that will give you a possibility of somewhere between maybe 50 to a few hundred bookmarks you can create on those sites. If you use proxies then you can create unlimited accounts and post thousands of bookmarks. If you really get good at it then instead of getting just one bookmark on a site like digg, you can actually get your site to have enough diggs to get noticed.
The same thing applies if you want to create multiple facebook profiles, craigslist accounts, forum profiles, twitter accounts, yahoo answers, etc. Google won't know whether you created all those backlinks from one ip or 50, but the websites your posting on certainly will.
Another thing to consider is how easy you want to make it for your personal ip to get spam reports. If you're spamming out backlinks to your website, then there isn't much you can do to keep the website hosting company from recieving complaints, but if that happens you can just get another host and move your website. If your using you own personal ip though, then you could end up getting complaints to your internet service provider. Depending on where you live, losing that account might mean having to settle for a much less desirable ISP.
The more blackhat the methods your using, the more likely you might want to use proxies. But even if your mostly using whitehat techniques there are still many uses for proxies.
Hope that helps clear it up a little for you.
