I'm also not sure what fragmentation has to do with anything. I don't think I've used a feed reader that didn't understand all current flavors of RSS and Atom, so it makes absolutely no difference what the webmaster decided to use, my news reader can figure it out.
It is a little bit annoying when the webmaster doesn't put the full text of the article in the news feed, and instead wants you to actually visit their site to read their stuff. I'd guess that they do that to make sure that you actually visit the site once in a while and might accidentally view an ad so they can make a few cents or they hope that you might see something else on their site you might be interested in or whatever. It also saves some bandwidth by not downloading the full text of an article if it turns out that I wasn't interested in it.
I can't speak for others, but as someone who hand codes my HTML, it is a non-trivial amount of work to convert the entire text of a page into an RSS feed, whereas it is easy to add the headline, datetime, and summary.
I get that it is pretty cool to read stuff inside an RSS feed reader, but doesn't fit with my current workflow. My site has no ads, tracking, cookies (aside from the ones imported by the odd embedded youtube video).
It rather depends on the amount of content the RSS _author_ includes in the RSS feed. There's nothing in the RSS/Atom protocol that prevents you from reading the entire article, but some website creators decide to truncate the feed content.
My RSS reader of choice, InoReader, has the option to download the original website which solves the problem. However, I have over 200 feeds and it's rare to find one without the entire content being included.