Many of the people asking about certifications are people who are trying to break into the IT industry, or people who are experienced but still can't find work.
Right now is an extremely difficult time to get a job, even an IT job. While many people are having a lot of success, there are also many people who struggle to find a job at all. Some businesses are desperate for talented IT workers and programmers, but many of these same businesses won't give anyone the time of day unless they have a 4-year degree, several years of paid experience in IT, and / or a bunch of IT certifications.
I myself have been looking for a decent entry-mid level IT job for over 8 months. I have a security clearance, 9 years of paid experience in a closely related field, 58 credit hours towards my A.S. in Computer Information Systems with a high GPA, a Security+ certificate, and 15 years of unpaid experience troubleshooting, repairing, and working with computers. I've written 50+ resumes and had them edited and critiqued by someone whose full time job is finding employment for people. When I was looking I lived in possibly the best job market in the world for cleared personnel (D.C. area) and I've applied to well over a hundred jobs, many of which I'm overqualified for. All I have to show for it so far is 2 interviews at companies that offered to pay me barely enough to support my family in positions that wouldn't give me the experience I need to get the job I actually want(took one of them) and another interview with a government organization that wanted to talk to me right up until I told them that I am a reservist.
I think that there's a lot of people out there in a similar situation, who see value in certifications because there are businesses out there who place such an important emphasis on them. From my perspective, if I ignored certifications that employers are looking for, then I'm not doing everything I can to make myself more employable.
A crap Linux cert will be crap. There are some good ones though, like RHCE. You can be Linux admin for years and still not pass without heavy studying. That's because there's a lab component to it.
Is it worth it? Maybe, if you're trying to sell your services, either in the form of a HR bypass filter ("Look, he's certified in Linux!") or as a consultant ("You should trust our team because we're all certified"). I know only 2 people with certs, and they fall into the latter category.
Most of the time though, I think it's not going to be worth it. If your current work will pay for it and you're willing to put in the work I can't see any negative aspects to having it, but I don't necessarily see the return on investment.