Welcome to BYC!
It is not necessary but wise. Once Merrick's is in your flock and on your soil, it is there for good. It can be brought in by wild birds or by you if you came in contact with infected birds.
Now for me, my birds may have been exposed to Merrick's, and I did not have them vaccinated. Because of this I warn everyone I sell to that there is a possibility of previous Merrick's infection, which has cost me a lot of sales. I would never lie to someone about bio security though, so I willingly take that risk.
Every two or three years I will loose a couple of birds to Merrick's like symptoms, but thankfully it is uncommon in my hardy flock. it is up to you. Merrick's vaccinations can be expensive to do yourself if you only have a handful of birds since most places only sell the vaccine in bulk. If you breed for hardiness and disease resistance like I do, then you may not want Merrick's, since the disease tends to kill the weaker birds. But if you are breeding for other traits, hate to loose birds to any disease, or just want a few chickens then a vaccine might be worth it. Either way it is your choice.
The vaccine isn't necessarily what one would call chemicals and harmless. Modern vaccines are little more than a basic harmless fluid (i think saline) and dead virus to teach the immune system to recognize the virus and kill it.
I think if I had, had the option to do it over, I would have vaccinated my birds.
