Subscriptions themselves aren't an abuse of IMAP, obviously, as they are in the spec. A client that *by default* uses them to hide folders is abusing them, for exactly the reasons I explained. They are non-portable because:
I agree 100% that hiding folders by default is bad, but I've never seen a client do that. I have seen many clients hide folders that are not subscribed, and that's exactly the behavior that I want. (If you don't have a subscription list, you see everything. If you have a subscription list, that's what you see.)
The interaction that most (all?) clients poorly bake in between subscriptions and the "IMAP root" setting means that if your various clients are not configured identically, you'll see one set of folders in one place and another set in another. This contributes to users thinking mail has disappeared to creating mailboxes with the same name at different paths. The latter is annoying to begin with, but becomes especially bad when yet another client shows the user both of two same-name folders and *resolves them in the interface to the same directory*, so the user thinks they are simple duplicates and deletes one.
Different clients interpolate names differently, such that even if two clients are identically configured when it comes to the "IMAP root" and namespaces, they map the subscription to inconsistent paths (either on the backend or the in the interface). This is especially true of moving between Thunderbird and certain versions of Outlook and Vista Mail.
Are these problems with subscriptions or folder handling in general? In
every client I've looked at, a subscription is just a visibility marker
for a folder. When the folder is visible, it shows up in whatever place
the client would put it if there were no such thing as subscriptions.
Are you saying there are clients that make these UI problem just for
subscribed folders?
Or are you maybe talking about subscriptions to public/shared folders?
If yes, never mind. I don't have any experience with client behavior
for those.