A resume with a "for everyman" cover letter is junk mail. A resume without a cover letter is used to line the bottom of the birdcage. Most direct mail hits the trash barrel between the mailbox and the house. All unexpected and standard resumes go from the IN box to the trash box. Some may generate a rejection form letter; most get ignored; 99.2 percent get tossed.
When a salesperson calls on a customer without an appointment it is a cold call. Cold calls have a low success rate. The customer may have absolutely no need for the product, may not even be in the office. Telemarketers who call at dinnertime have a low success rate. The customer may be too busy to talk, may have absolutely no need for the product, or may not be home. Resumes that arrive without invitation have a low success rate. The person who receives the resume may have no need for an additional employee, may not even be the hiring person.
You are the product, and your resume is your sales literature. Super salespeople never send literature before meeting with a prospective customer. They know that sales literature sent prior to a need analysis is odds-on to be irrelevant, off target, and unread. Super salespeople send literature after the first interview or bring it with them on follow-up calls. If the literature is not completely customized to the customer's needs, the salesperson highlights those product benefits most meaningful to the customer. Super salespeople create interest in their product and use sales literature to reaffirm and to leave a footprint, a product remembrance.
You must heed the example of the super salesperson. Your resume has much more validity and Velcro if the customer reads it after talking to you, hearing about you, or meeting you. This is particularly true if your resume is written for the hirer after you have interviewed the hirer. Your resume will validate your ability to deliver what the customer - the hirer - needs.
Turn junk mail into money mail. Don't send a resume without proper setup. If possible deliver your resume in person. Present your resume. Follow up with your resume.