That's a remarkably good point, and I should have thought of it.
On the other hand... bureaucracy can be pretty good at forgetting safety protocols and losing documentation, even over a period of less than ten years. We should definitely at least continue to make a big political point of storing them, so that the storage locations will be documented for the people who care to look it up.
Also, the people who are conscientious enough to bother to keep up the containment would be far fewer, in absolute numbers, in magical England (which needs to store Dementors) than in muggle America (which needs to store nuclear waste), due to the much smaller total population. If that absolute number dips below one, there could be pretty serious trouble.
Update: Discussion has moved on to a new thread.
After 61 chapters of Harry Potter and the Methods of Rationality and 5 discussion threads with over 500 comments each, HPMOR discussion has graduated from the main page and moved into the Less Wrong discussion section (which seems like a more appropriate location). You can post all of your insights, speculation, and, well, discussion about Eliezer Yudkowsky's Harry Potter fanfic here.
Previous threads are available under the harry_potter tag on the main page (or: one, two, three, four, five); this and future threads will be found under the discussion section tag (since there is a separate tag system for the discussion section). See also the author page for (almost) all things HPMOR, and AdeleneDawner's Author's Notes archive for one thing that the author page is missing.
As a reminder, it's useful to indicate at the start of your comment which chapter you are commenting on. Time passes but your comment stays the same.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: