Whenever a client wants an FTP storage solution, my instinct it to direct them to S3 instead, as a secure and cost-effective alternative. Is there any good reason NOT to use S3 instead of FTP/SFTP in any situation where you're setting someone up from scratch?
EDIT: To clarify: Is there any capability of FTP/SFTP (Outside of setting it up on your own server) that would constitute a compelling reason to go with an FTP/SFTP solution over an S3 solution?