I still say that they ought to allow you to suppress larger weapons, since IRL there's a guy who made a suppressed FA grenade launcher...
Also the same guy who made an integrally suppressed shotgun.
Someone else a fan of Red Jacket, eh?

I think most Police Forces in Canada and the US have at least AR-15s in the trunks of their squad cars now at the very least, as well as the shotgun in the front.
Depends on the department. Mine use either an AR or a M-4. I do know of some who have suplus M-14's. Some carry MP-5s but I would imagine those are more for urban areas. Rural, cornfield-type areas like mine sometimes require something with reach. Although, every deparment and jurisdiction has their own...opinions on having carbines and or rifles. Yes, after the North Hollywood shootout, many departments adopted the policy of a AR/Longrifle with a shotgun in the car. No one wanted that to happen to them.
But as to the usability of a suppressor will depend on the mission profile. Security, eh, maybe if there is a need to hide your position for whatever reason. Perhaps a few weapons with them but the majority regular. Military, especially special/covert operations, oh yeah. SWAT-style raids are kind of a waste as the dynamic entry tactics rely on a lot of noise and confusion when the operators enter.