Having an upstream can be useful for CI testing and for deployments with tools like Capistrano. Plus it allows you to pull down your code from other environments if needed and acts as a additional backup, so there are potentially lots of reasons to have one.
Because even though it's personal stuff, it's shared between many computers. For people that live in VCSs all day, it's just more convenient (history, branches, etc.) than e.g. storing it on Dropbox or similar.
Cloud access? I have a similar problem where my various machines are behind NATs/firewall (personal computer, workstation...). It could just be more convenient to put it in the cloud rather than have to poke port forwarding or VPN rules between the machines.
In my case, OS X has the latest git, while Windows is stuck on 1.9, and my git folders get corrupted if I do a push on OS X and simply do a status on Windows. I guess something has changed between 1.x and 2.x but this corruption is really not something which should ever happen in a VCS. My solution is to use Gitlab instead of Dropbox only local repos.
I'm not aware of any changes between 1.x and 2.x that would corrupt a repo... I use a mix on various systems, some even still have git 1.7.x on them. The only problem I run into is when you have some script that depends on a newer git feature added in a newer version...
My money would be on Dropbox busting your repo somehow. I don't think Dropbox is an ideal solution for pushing/pulling git repos from.
Assuming it’s for redundancy, why not just copy the folders as part of a normal backup?