So when you have 2 features/bug fixes in the same area, built in 2 different branches and deployed to isolated dev/test environments... the first time you might find out that they dont work well with each other is in Staging? Why wait for that when you could deploy to a common environment and catch issues sooner?

Btw... I am trying to get some teams in my company to get out of branching. So trying to understand your view (which is exactly what these teams are doing), hence these questions...