I think as Mary indicates above, you're going to get people who got certified and feel sure that it has helped, and people who didn't get certified and who don't think it will help. You'll probably also get people who got certified and wished they hadn't bothered, and people who aren't getting certified and wish they could.
The problem is that everyone either gets certified or doesn't and then can't compare the jobs they're offered against the jobs they would have been offered if they hadn't or had gotten certified. Jobs are different all over. I'm sure that there are some parts of the world where MSCs are desperate for shoppers and have jobs for everyone who wants them including uncertified newbies. And I'm sure there are areas where there is a glut of shoppers and not enough jobs even for the really experienced and certified shoppers.
The only way to know for sure would be to conduct an experiment where you have two shoppers who are exactly the same (same age, gender, income, education, zip code, MS experience, and who write the exact same essay upon application - which might be a dead giveaway) sign up for an MSC. One shopper should be certified and one not. Then check to see if they're offered the same number of jobs. This experiment wouldn't be much of value, though, because it would only tell you whether certification is valuable for that very specific location and at that time for that particular demographic. You'd have to repeat the experiment all over the world with different people and at different times of the month, year, etc. Sounds like too much work to me.
I have heard people who got certified say that while they don't know if being certified helped them get assignments, they found the training they took in order to obtain the certification valuable. They felt more confident taking those first few shops because they had some training and tools they learned. I think for really new shoppers, that might be valuable.
Shopper in California's Bay Area