Unless you've got tight control over the external storage than I would think security/integrity would be a big issue.
No guarantees that the file you are expecting actually exists and, if it does, whether it has been swapped inadvertently or deliberately.
No guarantee that someone hasn't changed the neccessary permissions when you need to add/change/delete a file.
No guarantee that the d***d folder even exists without checking every time......



Access to image folders is likely to be as tight as database storage. In most cases, many users will have view access and a smaller # will have add/change/delete rights.

Each of your bullet points has a counter-argument on the database side. In this context, it's zero-sum.

I think we've beat this one to death, but a good discussion. :)