This wouldn't work for 90% of companies. You cannot do whatever you want in a PCI environment, or a HIPAA compliant environment. It also boils down to your managers, product managers etc.. Do they micromanage? Is your focus shifting all the time because new high priorities tasks are coming your way?

If so, I would never feel confident in doing whatever I want, as I know I wouldn't be able to put 100% of my focus on it, and probably forget something important that could screw a lot of things up.