"There is nothing wrong with future proofing or just not worrying about space at all."
Not cost effective -- if you have need of say 2TB of space today why would you buy 16TB? So he is going to raid everyone assumes - so lets say 8TB he bought he doing 10 or 0+1, etc. Seems pretty pricey to store what exactly - home movies? Unless he plans on ramp up on his storage needs very very quickly 8TB if he 2 is quite a bit of growth.. Why spend $ per GB when he does not have use of the space, next month the price per GB is going to be lower, and prob the 6TB drives are out, etc..
Maybe he just is updating his 3TB drives because they are full, in that case ok.. Without knowing what his storage needs and use is currently it is very hard to say.
But I would not suggest someone buy large amounts of space today if they don't need it.. Drives are just going to be faster and cheaper next week, why buy space to sit empty. The drive is not going to last forever as well, so buying a drive that sits empty just so it can fail with 10% of is space used is pointless waste of money.