ZydoSEO - 2:25 am on Nov 20, 2012 (gmt 0)
The problems with blogrolls are that 1) they are part of the template (not the body of the page) and 2) they create LOTS of links from the same site and 3) with the exact same link text. And having lots of links to a page with the same link text is an issue now days except when it's people linking to your home page with your web sitename. These are the very types of links that Google is now working hard to detect, devalue, and possibly even filter your site for with updates like Penguin.
You are correct that links from blog post themselves tend to loose juice (and therefore their punch) over time.
When a post is new, the post has links from the home page, page 1 of category/categories, page 1 of the tag page(s) (if applicable), and page 1 of that month's archive pages... And the first page of all category/categories and page 1 of all monthly archives pages are typically linked to from every page on the site because they are sidebar widgets.
Once 10 more posts are made on the site, the post rolls off the home page and loses it's most powerful internal link.
Once 10 more posts are made in that month, the link to the post rolls off of page 1 of that month's archive onto page 2 of the archives. At this point the page loses a lot more link juice because page 2 is typically ONLY linked to from page 1 of the archive where page 1 on the archive is linked to from every page on the site.
Once 10 more posts are made in that category (or categories), the link to the post rolls off the page 1 category page(s) and onto page 2 of the category/categories. Page 2 of the category does not have links from every page on the site, typically only from page 1 of that category. So your post loses even more link juice.
yada yada yada
However, this same fading effectiveness phenomenon occurs with pretty much all unnatural link building techniques... article submissions, forum links, blog commenting, etc.
And therein lies what I think has always been the biggest issue with unnatural link building techniques. The effectiveness of those links built ALWAYS fade over time. This means you have to continue to build links JUST to maintain the same amount of "juice" coming into your site... not to improve rankings, but simply to maintain them. It's like swimming upstream. The more unnatural links you have in your backlink profile, the stronger the current and the more links you have to constantly build just to maintain the same amount of incoming "juice".
But decaying link juice from a guest blog post seems a better alternative than having URLs on your site or even your entire site filtered by Google because you have to many inbound links that are sitewides with keyword rich anchor text as is usually the case with blogrolls.
The holy grail remains truly editorial, natural inbound links given freely from webmasters of other traditional (non-blog) sites which tend to NOT fade over time unless the other site's overall authority fades.