1) Craig, where are you working that they are using 2x4 for rafters? Standard construction for ceiling rafters is 2x6 not 2x4. Pretty sure even in Canada that is the case.
​Otherwise how are in-ceiling speakers suppose to work at all. They build them that depth for a reason, although you can get special 'shallow' mount ceiling speakers for that rare occasion.

​2) Building back boxes for in-wall / in-ceiling speakers is quick and easy during construction. I do it all the time. I'm perplexed by the others that say how time consuming it is?
​Simply cut 2x4 or 2x6 blocks (depending on wall or ceiling), the scrape wood is usually laying around the jobsite. Slap a 1/4 sheet of ply or OSB on top of ceiling locations (wall locations will automatically be sealed by the other side). And I use liquid nail around all the seams - you could also use silicone but liquid nail dries faster and not as much odor.

​3) The main purpose for doing this, actually has nothing to do with bass. What we are concerned with, is unequal airspace between 2 speakers. Causing a slightly different frequency response from each. That will hurt stereo imaging and sound continuity. Building back boxes insures the same frequency response from each speaker. Also allowing consistence bass response, instead of multi resonant bass frequencies. This actually is more important for in-wall applications, as most ceiling locations tend to be infinite baffle as far as VAS goes.
​But another good purpose for back boxes is sound transmission to other unintended areas - which actually becomes more important with ceiling locations sometimes.
​And yet another reason is for fire block rating. If you already are use to making back boxes for all your in-wall / in-ceiling speakers; then when you run into a need to have to make them for firewall ratings, it is not a big deal.