I think spam is spam no matter who sends it (engineer or recruiter). If I send or receive a random message, it'll be just that: a random message in an inbox. Such messages are usually ignored. While recruiting for my startup, I've had the most success through introductions rather than sending random messages to people.
Interesting, I think you highlight the point that the more personal the contact is, the more fruitful it will hopefully be (see purereason's reply). While you consider all recruitment email to be spam, it seems that not all do (according the results of the experiment shown...)
