What does it depend on?
Define "need"..
Go for it! To those of us in the USA, Ontario is in a foreign country.... have pondered awarding myself a self-conferred arborist certificate (with an embossed gold seal) and a degree from an foreign university as well.
A Arborist should be able to do all tree work. Period.
Word, gotta love it when I bid a maple and another dude calls it a oak, LOL! Or when they miss diagnose, treat it for something that it does not have, the tree dies and they say "we tried, it was too far gone" after they killed it. A few years ago, while I was off playin Jarhead, A long time client wanted to know what was going on with their white oaks. They call a big show. They sell them on DRF, nail them with a absurd amount of nitro. Didn't check anything, just looked and sold. Burned them so bad that the leaves where crunchy. Trees where slightly chloratic, something a little sprinkle on top could have taken care of. When I came home, he called me over. I could still smell the nitro, ended up removing the 4 and replacing the turf. I wanted to do a soil sample but HO didnt want to pay for it. Always wondered of what was really in the tank. Their fert is supposed to be so good that it can never burn? Figured that is was a contaminated batch. I know they like to use the same truck for everything. Doubt they do much cleaning.I have never gone after the HV stuff either. Working in close proximity to primaries gets me stressed out way more than climbing something that is patently unsafe.
I let my Ont. pesticide cert ("landscape exterminator" or something like that) expire and couldn't be bothered keeping it up.
Big problem I see with (some) folks who actually get certified is that they oversell their abilities. They misdiagnose problems. They screw up tree ID. They prescribe "deep root" fertilization without bothering with soil sampling. That kinda stuff gives the arb industry a bad rep.