Today I was going through some bookmarks and I realised how many regularly updated websites I have been completely ignoring because they don’t have an RSS feed. There’s a set of sites I keep in my "daily" tabs in firefox/mozilla – that is to say when I fire up mozilla or firefox, I click my "usual suspects" bookmark and all the sites I visit every day open up in tabs.
The last of these is my blogs at bloglines.com, which I keep open all day and use like an email inbox – bloglines goes off and downloads all the RSS feeds I have subscribed to and I can see at a glance which ones have been updated (note that when I am logged in, bloglines keeps track of which ones I have read). I’ve become so used to this way of browsing the web that I tend to forget that I have any other bookmarks.
Some people don’t have RSS feeds on their site because they don’t know what they are or don’t want to/can’t set one up. Some people don’t need one because they never or rarely update their sites so the user only needs to visit it once. Some design oriented sites don’t have them because they don’t like the idea of people reading their content without seeing it in the context of their designs, although the solution to that is to only publish headlines and summary paragraphs so that people click through to the site to read the whole article.