Atleast from what I've heard it is, but IMO a search engine ranking can get you visitors but good rich content (such is the way of AJAX) will keep the visitors coming, word of mouth is cheap and good advertisement.

Although AJAX can be used for things non-related to a search engine such as

- Form validation
- Cool little shoutboxes
- Slide shows
- Can't think of much more, but I know there is more.