That is a good point, and I agree, it's more the name on the endorsement than the actual endorsement.  (I sometimes skim the names of the endorsements to see if there's anyone I know.  If there's not, it means nothing and I don't bother reading what they actually said.)
Practically speaking I myself would be wary about writing an endorsement for someone until I've got a bit more of a track record.  Yes, I've got an established readership, but until it's a little larger I think putting my name on someone else's books is ostentatious at best.  I would still pull the "Aubrey likes it, so I'll probably like it too" card through blog posts and things, but I wouldn't consider my name worthy for an endorsement just yet.  I have been asked and declined for the above reasons, plus I don't think new self-published authors necessarily need endorsements on their books.
I did write an endorsement for Jordan's logline book on request, that was to be used on the website.  I actually think I have enough klout to make that worthwhile because it's more of a testimonial.  I'm a professional author and I can truthfully claim that I rely on Jordan's help for my loglines.  That means something, more than "I'm another young indie author that likes this other young indie author's book."
I don't know how it happens in the real world... but introductions are a bit different in my mind.  I don't have much experience with them, except with the poetry book and your Sanctity of Life series.  For the former the introduction was partly to get Jay's name on it, which was important not only for marketing but also for a sense of ownership/unity since he's the founder of Holy Worlds.  It was like getting his stamp of approval.  For Sanctity of Life, the introduction was originally more of a practical thing, to literally introduce and explain the series rather than just endorse.  I think it helps round out the series by giving a kind of "Here's what this is" before diving into the posts.  You could have easily written such an introduction yourself, but I do think in this case having another author write it had a nice effect.  It distances the introduction a bit since it's coming from someone who is not presenting the deeper, impassioned material of the main posts, and when it was on the blog it afforded some sense of completeness because I was the blog admin.  Overall it just felt very suitable for the book.  But in both cases, the writer of the introduction had some kind of personal ties to the work, rather than just "I read it and liked it."  To me an introduction has more meaning if the writer has some kind of personal experience with the work and isn't just writing an "I love this!" piece.  Even "I know the author and have watched this book develop for years" is more personal and meaningful to me.
And I'm kind of just thinking out loud here.  It seems to me that introductions are more for nonfic, and I don't always read them.  

  (I don't always read the author's foreword, either.  

)  
What are your thoughts?