I appreciate that they don't delete files because this lets them offer some convenient features related to backup, account history and uploading. The biggest reason not to delete concerns sharing. If I share a folder, share a file, then a user deletes the file from the folder, I want to be able to retrieve it as I've sometimes had to do. I don't want someone else to be able to irretrievably destroy it on the Dropbox server.
However, I agree that users should be able to delete a file from Dropbox' servers if the user, since the beginning of the file's life on Dropbox, had the only account possessing the file on Dropbox. The procedure should involve a lot of red warning buttons and re-entering of the account password. I think this should be possible because some files really are too sensitive to be shared, and I can see how they might accidentally be pasted or dropped into the Dropbox folder and instantly duplicated. If the file is dropped into a shared folder, then there is probably nothing to be done because obtaining multiple permissions to permanently delete a file would be so complex socially.
However, I agree that users should be able to delete a file from Dropbox' servers if the user, since the beginning of the file's life on Dropbox, had the only account possessing the file on Dropbox. The procedure should involve a lot of red warning buttons and re-entering of the account password. I think this should be possible because some files really are too sensitive to be shared, and I can see how they might accidentally be pasted or dropped into the Dropbox folder and instantly duplicated. If the file is dropped into a shared folder, then there is probably nothing to be done because obtaining multiple permissions to permanently delete a file would be so complex socially.