It's amazing how much stuff you have to know to take the certification exam, but how basic the stuff actually is when you get down to it. Just imagine if an arborist didn't know any of the information on the exam!
At least the certification gives you a benchmark, otherwise you go off reputation alone.
I believe reputation is more important than the certification, but if you don't have a reputation

the certification is going to do a lot.
If you take two arborists, one certified

and one that is not

, the one that is certified is not necessarily a better arborist

...that individual's knowledge has been measured by a standardized test

, while the other has been measured by their reputation alone

. The reputation is what measures whether the arborist is trustworthy

, reliable

, knowledgable

, skilled

, etc, or not

!
The only addition I will make is that the reputation is often built off a client base that has no clue how to take care of trees, so who can say that reputation accurately measures whether that sweet talking arborist knows anything about pruning etc. The certification measures your knowledge of actual arboriculture. So, I guess you can say the certification gives you a more accurate picture of the arborists base for that knowledge...but might not demonstrate the way in which they use or retain that knowledge.
For the longest time I didn't think the certification was necessary!
This is all coming from someone who will be passing the exam on Saturday!

...don't worry I have a great reputation too!