It's been a couple of ArM supplements I'm not fully convinced by story seeds. I find many of them too straightforward (i.e., I thought of the idea myself just by reading the text). I think now, I'd personnally prefer full stories instead, even short ones. As a SG, I wonder whether it's easier to adapt a full story or develop a story seed from scratch (I mean in general, I've already stated my preference). I'd really, really prefer one nice story per sourcebook instead of many dull seeds. Am I alone? 
I think the story seeds are a good idea as long as they give more material than the main text. Simply stating the obvious way of using an story element has no interest to me either.
More than a fully developed story at the end of each book I would like to have different short (one page?) stories linking together the different elements provided in the main text with new ideas showing how to make them interact between each other or with the players. It could also be separated between saga and adventures stories.
I think The Lion and the Lily final chapters are quite good from this point of view, even if some of the saga ideas are not detailed enough.
I like the story seeds. They spark interesting ideas. I've used several and have a couple I'm kicking myself for not using.
One consideration I make when adding story seeds into anything I might be working on is the knowledge that players and storyguides alike are probably reading them at much the same time. So I tend to shy away from straight "Here's the problem and here's what's causing it" story seeds in favour of "Here's the problem, but is it this that's causing it? Or that? Or something else entirely?"
I think/hope that sparks the SG a little more. The problem is going to be apparent to the players thirty minutes into the session whatever you do so I'm not worried about concealing that so much, but even if they've read the same story seed as the SG, they shouldn't know (from the text) what's causing it or how they might need to resolve it.
And I wouldn't make too many assumptions about what's obvious from simply reading the text.The story seeds do a nice job of drawing the eye to possibilities that some might not at first consider.
I can understand why more full stories aren't published. So much of Ars Magica stories is YSMV writing and marketing "generic" adventures is very difficult, I imagine.
As someone new to ArM, I find the seeds helpful for answering "how could that be used?" questions, particularly more obscure subjects not usually featured in RPG's ("how to make an adventure about bookbinding"). I also tend to think of this as "filler material" and am not expecting perfection from the sidebars. Still, I often feel like the seed is not fully germinated. A few more paragraphs, and I'd have enough to "grow" into a full story.
I haven't used much of it yet, but I liked the approach of the Hermetic Projects and Ancient Magic books, which gave "story saplings" if you will - a bit more information, but in the form of a dozen or so campaign themes and/or elements, each with locations, individuals, seeds, plot ideas, major events, spells, items, etc, related to that theme. Very useful. I'd love to see a book along these lines, even a bit more developed, describing some mystic places or creatures.
Based on the open call-for-writers of roughly a year ago, there's likely an upcoming book that focuses on stories designed to tie in with the various sourcebooks. I'd expect that to show up either late this year or more likely next year.
That said, I both like and loathe the more developed stories. I like them because they give me lots of ideas, but I loathe them because my feeling is always that chances are players have read them too, somewhat spoiling the surprise.
I liked the format of Legends of Hermes more than Tales of Mythic Europe mainly because LoH included a bunch of 'how to hook your group' preludes, while ToME just had an adventure dropped in.
I'd love to see a 'saga guide' or similar - a not-quite-fully-developed saga that is a great starting point for a new group, or as a source of ideas for an established one.
I agree with humboldtscott's comment; a lot of ArM stories are YSMV.
In particular (I feel anyway) really good ArM stories are about what the player characters do and why. Unfortunately, the ArM authors can only have a very generic idea of who the player characters are in a story. This is unfortunate, because the more developed a story is in the supplement, the more it tends to be about what the NPCs do (because that's who the author knows about) and the less it tends to be about what the player characters do.
Next time David asks for playtesters, volunteer.
I don't prefer a single story, because I don't know your PCs, and since most Ars stories are meant to come from the Virtues, Flaws, Boons and Hooks for your group, it makes generic stories quite hard to write.
As to the obviousness of story seeds: yes, I agree. They are often really blatantly obvious. The reason for that seems to be to make things easier for people new to roleplaying. I know when I just mention things off the cuff they come back from the playtesters with a request to mark things as story seeds. The problem of course is the pretty much everything is meant to go into a story, so a book which is set in the standard settign and does not have extensive new rulesystems is, in some cases, pretty much all story seeds, so there's no point in marking them.
That would be my preference - just writing text on the assumption that everyone knows that everything is a story element - but its not going to happen, because its not about experienced GMs like you and I, its about developing the skills of people who are less good at looking at a bit of text and saying "What could I do with this?"