Some of the debate comes down to format. Many recruiters and hiring managers have no desire to open a bunch of attachments from an aspiring employee, so if your email isn't your cover letter, it's not getting read. They'll read your email, open your resume and that's it, unless they've asked for more.
Other people say email cover letters are prone to mistakes, and the well-regarded format for the formal cover letter is best served as a separate document. What do you think? Let us know in the comments below.