I've just been quite surprised to click on a highly ranked Google result for a Stack Overflow question just to end up on the search page, because the page doesn't exist anymore. I figured it might have been migrated to meta, and so it did. Trying a few others yielded similar results, so to illustrate the point I might as well pick a random example.
Admittedly the offered Google search might usually guide to the new location, but this could take time for lesser questions, plus requires more precious clicks of mine anyway. Smaller sites would also have to worry about SEO here, but this might be mostly irrelevant for Stack Overflow given its excellent base score in this arena. A more serious problem of course are broken links elsewhere, be it on web pages or with user favorites and the like.
I realize that there had been some related discussions regarding whether to keep stub pages for merges and duplicates or delete migrated questions but wasn't aware that the latter has just been implemented in the meantime.
Still it puzzles me that this extremely useful functionality has been omitted - providing a proper HTTP response status code 301 (Moved Permanently) should be easy, shouldn't it?
I'm marking this as a bug rather than a feature request, because it's actually breaking the web: Cool URIs don't change ;)