I believe that apart from some exceptions (such as “links” posts) each post should stand alone. A reader should be able to read a single blog post and understand the author’s point without needing to visit any external sites.
A common mistake is to write a post that can not be understood without following the links. This means that if one of the links gets taken down then the post can not be interpreted. Also if a reader has Internet access problems that deny access to the other site (which is not uncommon) they will be unable to find the original source and thus miss the point.
It’s quite common for people to download copies of blog content before going out of net access (I routinely load a Planet feed of the blogs I read before travelling). Some people read blog content via email, for such people reading blogs without net access will be even more common. If a blog post can’t be immediately understood then a significant number of readers will just skip it. If too many posts from one RSS feed (where “too many” is a subjective value that varies from reader to reader) have this problem then they may just unsubscribe from the feed.
Also even people who do have good net access will sometimes skip posts which require them to visit an external site. It takes more time and if they aren’t sure that the content will be of interest then they skip it.
Finally writing an explanation of your point tends to result in more clear communication. At the shallow end of the blog pool it’s quite common to see posts which link to web pages and express disagreement with them. If the web page which is referenced makes several points (it’s very rare to find pages which strictly make a single point with no sub-points and no chain of logic to support the point) then it can be difficult or impossible to determine what the blogger specifically disagreed with. A post which summarises a page and gives specific reasons for agreeing or disagreeing with it gives little potential for confusion or miscommunication.