I remember thinking there's no way I'll ever fill the 2TB HDD I got when I built my PC. Here I am a year later and it's more than half full.
I've currently got around 6-7TB of data across a few different sized drives, smallest being 64GB and largest 2TB, and it really is a pain in the ass to manage and I'm always worried a drive will fail and I won't have a backup of something. Looking forward to these 8TB drives making it to retail, I'll probably get 2 and setup a RAID1.
If you put all your data from different drives to one big 8TB one you will probably worry more. After all if that big disc will fail, you will lose everything in one go. Isn't it better to keep your data on different drives?