Certificates are most useful when 1) your resume does not clearly show extensive skill, 2)the person reading the resume does not know the material.
Unfortunately a lot of resumes get filtered out on keyword matches. I was once filtered out by HR for not explicitly saying '.Net Development', verses what I had 10 years of 'C# development'. Only found out because my friend at the company hunted down HR to ask why I was not being interviewed. Neither HR nor the hiring manager understood that C# was .Net.
If the job you are looking at is a shop with a lot of people with the same skill then you do not need it as they can tell from the interview.
If it is a company with few people, or they provide your skills as a service, then yes you do need the certs.
Is this clear? Seems like I have been rambling here a bit.
But in my experience professional networking brings the best results - no questions about certs or silly coding quizees.
Best people I know haven't done any certs. Most people I know who did certification had not much prior experience in a subject and rarely had ocassion later on to gain one.