Cross contamination can be an issue. If you have three healthy fish and one fish with parasites you'll end up with four fish with parasites.
Another issue is compatibility. If you QT aggressive fish with passive fish it typically won't work. However if you qt fish you want to pair or schooling fish having multiple fish in qt is a good idea.
Kind of defeats the purpose. If you have one sick fish and one that is not sick, there's a greater chance you will wind up with a sick pair. I also think it's a good idea to run a QT fallow for a few weeks after QTing a fish.
Assuming they are compatible and your tank is big enough, yes. If you are starting a tank and trying to stock it, you are almost forced to, since you couldn't add fish any faster than 1 every 6-8 weeks otherwise, depending on how long your quarantine for and how long you let your QT sit empty.
As mentioned above, the down side is if one develops any symptoms, you're obligated to treat both for the duration. Even though you expose the 'healthy' fish to the sick fish, you still prevent the exposure to your DT.