First reply is best reply? Seriously though, pukunui essentially hit the nail on the head, the PHB backgrounds are just examples to give players and DMs an idea of how to put them together. They're essentially a small bundle of extra proficiencies, a few trinkets and bit of a quasi-mechanical ribbon to give a characters backstory a little more weight in the game.
Now my players have been just using the ones found in the PHB and SCAG exactly as presented, but I've made it clear to them that of they want to tweak them or completely build a new one from the ground up that they're more than welcome to as long as they let me look it over, to familiarize myself with it if nothing else. As I said though, none of them have done that as of yet
For myself though I've modified some of them when creating NPCs using the normal character creation rules. As a simple example, on NPC had the Sailor background but was the ships cook, so the proficiency in navigation tools didn't make much sense to me, so that was dropped in favor of cooking tools. I also, odd as it might seem, completely dropped the proficiency in "water vehicles" and gave him an extra language instead to represent less time spent sailing the actual ship and more time spent with locals at exotic ports.
Either way, whether you're talking little tweaks or building new ones from scratch the backgrounds are pretty easy to mess with, which I think is great.