I think you're exaggerating. Parisians may tease Quebecers, but that's just to keep themselves in teasing shape. Teasing is a sport, you gotta practice. Most French people will be able to understand most Montrealers.
It should be noted that France itself has many regional accents that are quite strong.
Also to be noted, there is a steady flow of young French people immigrating to Montreal.
Most French people will be able to understand most Montrealers sometimes, in the same way that someone from Britain would be able to understand a Cajun sometimes. The language is very divergent, with franglais words sprinkled here or there, shortened words, differing word usage, etc.
On top of that, for the reasons above as well as others (including their accent), people from France really dislike listening to people from Montreal speak 'French', and will avoid it whenever possible. This is similar to the way Francophones from Montreal will speak English rather than French to an Acadian, because their accent is really just that bad.
You seem to be living in Canada, but perhaps somewhere else? Our views are pretty different.
I'm a francophone, born in Quebec. I have never seen a francophone (from anywhere in the world) speak in English to another because the accent was too strong. Most francophones in Montreal actually speak pretty bad English, at least once you leave the technical spheres, and any Quebecois can speak something closer to "international French" when he knows an European is listening.
A large portion of my classmates at university were French people. They had no trouble understanding the local accent. They might be puzzled by some special expressions we have for snow, for "take a chair", for shortened forms ("je ne le sais pas"->"jelsépa"), swear words, etc. but nothing that can't be explained fairly quickly. The difference between French from France and from Quebec is important but I'd say comparing it to Cajun is exaggeration. It is not my impression that French people dislike Montreal French, but rather that they find it amusing at first (it sounds like a rural accent) and eventually just get used to it. Many Quebecois humorists and singers are popular in France as well.
Franglais is a large debate, but in short, the French are as guilty of it, if not more than the Quebecois. Many words like "email" or "shopping" tend to be said in English in France but in French in Quebec ("courriel", "magasinage").
Well, my sources for my comments on French vs. Quebecois consist of ten or twelve French individuals I've spoken with on the topic (whose views were unanimous, and who spoke for the people they knew at home as well). Likewise, my views on Quebecois vs. Acadians are taken from an Acadian friend who had moved to Montreal, and whose coworkers refused outright to speak French to her because they didn't want to listen to her accent.
Put another way: I've never met someone from France who didn't speak ill of Quebecois French when asked, and I've never met an Acadian who felt entirely welcomed in Montreal. They could be exceptions.
At least 3 quebecois have told me they have had a french replying to them in english (in international contexts, like a conference). I guess it is only a reaction to an accent many of them have not heard before, but after a second they understand they are talking to a french-canadian
Sure, maybe two farmers from rural areas of each country won't understand each other, or a second language speaker who speaks in one accent won't understand the other, but otherwise it's simply pretentiousness.
It should be noted that France itself has many regional accents that are quite strong.
Also to be noted, there is a steady flow of young French people immigrating to Montreal.