I like this: tl;dr if it'll take the company more effort to reply to your email than you put into sending it, you're doing it wrong.

I don't particularly care whether it is an employer's market or an employee's market. My rule of thumb is that if your email is going to jobs@foo.com, you can send anything you want. But if you are sending an email to a person by name, it is a matter of courtesy to write a personal note.

Likewise, if that person replies to you from their personal email, you deserve more than an obvious form letter, even if it's just a single sentence.