This seems to be coming up a lot as users upgrade to the newest version. I understand that if you set all the users to the same limit as the site, then that space is shared up to the site limit, but to me it's just easier to be able to set the user limit as -1 and have the same results.
If I have to set the user limit as the site limit, I have to go and look up the site limit each time I'm adding a user, and each site has differnt limits, at least on my system.
Also if I change a site's limit in the future to give them more space, now I have to go and change EVERY user's limit again to match the site limit.
To me, if they are just left -1 and all spit the site limit, that just seems much easier to maintain. If someone wants to set a hard limit for a specific user, then that's when we should need to go in and change it.