So that he can still login simultaneously with a FOB (Wow Security Device) is why he's going with 8 separate Bnet accounts,

There is little current drawback to having 8 accounts other than having to wait the 60 days before being able to transfer between accounts.

Drawbacks
1. Loss of any benefits of having your accounts on one Bnet (as they start adding them other than the easy transfer of Characters)

2. That at some point in the future that they may still end the Simultaneous Login with a FOB