A thoughtful question you have here. A trackback URL is used to notify a third party that they are being mentioned or otherwise referenced somewhere else on the web. In the context of social bookmarking, the trackback URL allows another party to know that their content has been bookmarked. This is similar to the CC function in an email message.
In general, they can be very effective in making a website an authority when used correctly.
They are not normally permanent. Getting some is a decent indexing method but as the URL's they are on change/move/get deleted they don't have long term value, but useful all the same.
GSA can churn them out like there is no tomorrow. On default settings and with a big enough keyword list you can go ballistic on this sort of indexing resource.