There are different strains of MG, some less pathogenic than others, even as there are different strains of marek's. It's quite conceivable you could vaccinate against one strain but still acquire others.
On rare occasions it's also possible for *certain* vaccines to spread disease. I don't know about the MG vaccine and don't want to speculate, but the basic rule is, if you vaccinate, you have to vaccinate forever.
The example I'll give for spreading of disease is tick fever in cattle. Here in Australia, if you use tick fever vaccine with cattle, the local ticks will bite the cattle and become infective if they then go on to bite any non-vaccinated cattle. My local Department recommended I vaccinate for tick fever even though it was unknown in my region... That would have ensured that it soon became known in my region, which I found astonishing for an agriculture department to advise doing. (It begs the question whether authorities and vaccine distributors are too closely linked these days.) I doubt whether some sort of infectious result is true of vaccines for CRD, but it's worth thinking about before committing.
Ultimately though it comes down to what you read, what your worldview is and what your future plans are... I would never say someone shouldn't vaccinate or that they're wrong for doing so, it's a choice. Let's hope it continues to be one, because diversity of ideas, like diversity of genetics, is the best way to ensure long term survival for a species.
The way I look at it, biosecurity against MG isn't difficult, and given its short lifespan off the bird it's also fairly easy to eradicate if you can source MG free birds (which you can if you're careful). Culling and cleaning out after an infected flock only has to be done once if you keep up an anti-MG biosecurity plan (which again isn't hard)... Contrast this to ongoing vaccination of each and every chick, with possible pathogen mutation to become resistant to the vaccine, and the case for vaccines starts to leak...
cheers
Erica