If you don't have any other information available certain test results may be the only thing you can cling to.
If I were to work with a new colleague tomorrow, I would want to know what his or her professional background is. I'm not one of those who expect them to have a university degree but if they don't they would have to show me some other proof. I'm not going to risk my own reputation by teaming up with somebody who maybe just spent two weeks in France before signing up for the job. Since our profession is a "non-regulated" one, meaning that anybody can call himself an interpreter and/or translator and can work as such without having to provide any diploma and/or information on his professional experience, I need something I can base my judgment on. And, believe me, I have had cases where clients hired "interpreters" (especially for English) who had spent a couple of months in England as an au-pair and all of a sudden they had turned into "simultaneous interpreters". This mostly happens when clients want to save money. They sometimes even ask their own secretaries to do the job - luckily for us, those poor guys fail, which is quite understandable. Not because they need to have some university degree but they need some kind of training and/or experience.
If somebody spent 5 years at a university and was formally trained as an interpreter, I can at least assume he has understood the basics of the trade.
As for the rest, it all depends on many different factors. His or her experience, the way they cope with stress etc.
Basically, however, it all comes down to what you are able to do when you are in the booth and working.
I would not just trust anybody's words who I have never met before. Here I totally agree with JayB.
I need to see and hear for myself what somebody can do. And until I have the opportunity to do so, exams may be a reasonably valid tool to get a first impression albeit an imperfect one.
As for the languages I study as a hobby, I will never take any exams because I don't care if anybody thinks I'm at level A, B, whatever.
To me exams equal pressure - it might be different for others. I love language learning so much more now since I don't have to take any exams anymore. Besides, my levels vary because I don't have the time to consistently work on all my languages. So, taking an exam and then failing would just take all the fun out of my learning process.
I prefer talking to people in real life. And yes, I'm very much aware of my mediocre level in quite a few languages I study but that does not stop me from enjoying them. Actually, holding a simple conversation in Mandarin and/or Japanese (with all the mistakes I make) gives me much more pleasure than interpreting at a high-level international conference in one of my working languages. I like both but I find the first experience much more gratifying. Getting back a smile and some nice words from the Chinese people I talk to in my hometown is a much better motivation to me than any test result ever could be.
If I ever were to use Mandarin professionally, I probably would have to take some exams. But since this is not the case, I'll just keep enjoying my "test free" learning environment ;-)