I think id really depends on what you plan on doing with it.
Do you want it as a pet or to breed?
Alot of people who want one as a pet want a baby BP so they can watch it grow, and it grows up knowing them. Yes you will have to buy bigger items as it gets bigger. But you will always be able to use the old stuff if you get more.

Now some people that breed want to be able to breed that upcoming season and dont want to wait a year or two to breed. So they will buy a bp that is ready to breed or will be by the time the breeding season starts.
There is nothing wrong with buying a bp adult as a pet either. I think it will just be a personal decision.