SQL Server Service Account Configuration - one account per Server?
Separate service accounts for each server and for each service will restrict any issues to just one server and just one service in the following cases:


Service account getting compromised
Service account getting locked out
Service account password changed
Service account getting disabled/deleted/renamed by mistake


Imagine what would happen with a single service account if you entered the service credentials wrong a handful of times (scripts are great for this): you would lock out the account and tear down the whole SQL Server farm. Not good.