Essentially, you stay away from it because of the low signal to noise ratio.
For employers, the deluge of candidates who think that technology is HOTHOTHOT means that they have to look at far more resumes to find people who are worth bringing in to interview. Skipping Craigslist, for many companies, acts as a low-pass filter - the bet being that you'll ignore more noise than you will signal. After all, the good employer probably only needs one good employee, not 100 mediocre ones (especially in engineering.)
For employees, a similar phenomenon occurs. Once you've been burned once or twice in your career, you start to notice that managers at worse jobs tend to rely heavily on Craigslist, Monster and the like. These companies know that technology is HOTHOTHOT, but they not be savvy (or even care about technology) enough to go to meetups and post jobs in ways that engage the tech community. So again, many of the best employees will only use craigslist as a last resort, because they don't want to deal with the poor postings. Ignoring it is a low-pass filter for employees as well - after all, they only need one job.
There are plenty of more tech-specific sites with "elite" job boards, where the sheer knowledge of knowing about the job posting site - and understanding the technologies and discussions around them - is enough of a filter to encourage both good employees and good employers to show up. (Hint: check out HN user whoshiring.) But at the end of the day, the best jobs are almost always filled the old fashioned way - by word of mouth. And that's why it makes sense to get out there and meet people early and often.
(In a happy coincidence, it also turns out you'll learn a lot more in general by meeting and talking to people and frequenting tech sites instead of sitting in front of Craigslist.)
Obviously this stuff is wild generalization - and as with any low-pass filter, you take a risk by ignoring Craigslist. I usually go to Craigslist once in a while myself. In my experience, it's worth checking on them once in a while (if only to see what you'll find), but not relying on them.