Thanks for the offer, but...

Four years ago, I got annoyed with repeated emails from Google recruiters and asked them to place "DO NOT CONTACT" on my file. This worked for a few months; then I started being contacted again by recruiters who "just wanted to check in case [I] had changed [my] mind".

Asking for this to be changed to "DO NOT CONTACT ON PAIN OF DEATH" seems to have worked, though.

> And that's where things fall down. If someone currently has a job, then emailing them out of the blue to ask if they want to come and interview for something vague is hoping that they're either looking, and just haven't come to you yet,

Agree with this point, but

> or not looking but unhappy enough with their current role that all they needed to start was an email asking them to submit a recent CV.

this is also valid, but not addressed in OP. Consider an employee who is working at a large corporation or consulting firm, who is talented but not particularly attached to his/her employer. There are many of these people. This email out of the blue from Google (or whoever) is pretty nice, and really could be the impetus to make a career move.

If in-house recruiters are bad, what about headhunters?

I've received emails from a headhunter 3 times within a month asking me to apply for the same position. After the second time, I notified them that I was perfectly happy with my current position, and was not interested (noting that I had already respectfully declined the week before).

They didn't respond.

A week later I receive the same exact email.

Of course none of this trumps a friend of mine who was asked if he was interested in a "fantastic opportunity" at the company he currently worked for. After asking if he would get a referral bonus for recruiting himself for a job he already had, the recruiter hung up.