Documents in many places complicates things. For example, if you have to go through discovery, all of a sudden you have to make sure you find things on every single service you've stored them. That's (part of) legal's job, and the number of different services you're using is a multiplier on a lot of things.
What gp described just seems inefficient in general.
I won't question the (proper) assumption that enterprises typically use Exchange, but I will say that this is why solutions like Dropbox exist and are adopted in enterprises: because it's easier to get the information where it needs to be than what "enterprise" solutions offer.
What gp described just seems inefficient in general.