Most companies around these parts use similar services for resume intake, and those that don't seem to use email.
For the former, I haven't seen a resume handling service that doesn't have a "Cover Letter" text field, so if we can stipulate that those are common enough, why would an applicant leave it blank?
For the email case, it's a bit cold and impersonal to send an email with a resume attachment or URL with only whatever few words are necessary to keep your email client from popping up a warning questioning whether you really want to send an empty email.
