Another unintended impact of even unofficial time periods for callings, is the associated internal pressure placed on the member. If they don't serve the "intended" amount of time due to personal reasons, it's all too easy to feel like your service was incomplete or a failure. Likewise, there are way too many members who have silently struggled in callings for much longer than they should have, because they were under the assumption that they "needed" to serve longer.
Also, I may be more prone to this than others, but I can tell you that even in church callings, if I know my time is nearing an end, it definitely impacts how I approach my responsibilities. And when I've had callings where my release was a 'surprise', I definitely was more focused on things than I would have been if I knew I was being released soon!