Extremely Large Volume - Pro and Cons

"Extremely Large" is only meant in the 2013 standard.  I'm sure it will be "not so large" in 5 years and "tiny" in a decade.

We're a large organization and one of the responsibilities of my group is to manage about half a million user homefolders, which are currently spread across about 100 volumes in 10 file clusters.

As grand and majestic it may seem, problems arise when those volumes are filled up.

At one point the situation became so bad that I had to assign one of my staff to move homefolders around FULL TIME, in order to continue some service.  What an expensive and completely meaningless excercise to do!

We do implement quotas and expand the volumes regularly, however it never catches up with the demand, since we're forced to overprovision due to our size.  Imagine if we provide 1GB of space per user, quite pathetic in today's standard, we need 500TB of space just to fully provision the storage.

I'm aware of some technology (Storage Pool in 2012, EMC, etc.) for building very large volumes.  This will be ideal for my situation.

However, I also heard some concerns from different people in regards to extremely large volumes.  Backups, RAIDs and maintenance are some concerns, although I don't quite understand much in those areas yet.

Now, although technically possible, what will stop me from building a 10PB volume, put everything there and don't worry about it for the next 5 years.......

Pros and Cons, experts?