Just tagging on to this because my company is dealing with a huge battle right now with Person Accounts and the Marketing All Contacts list.
According to Salesforce Documentation ANY Contact, Lead, User, or PersonAccount Data source will be counted against your contact limit: Documentation (see final paragraph)
What this means is that is that you are effectively getting duplication if you are using the Account Synchronized Data Source as Salesforce set it up for you.
- Disable Contact Sync
- Add a formula field on the PersonAccount record for the CaseSafeID of the contact record
- Sync that field over in Marketing Cloud and drive all process off that ID as appose to the PersonAccountID
Which really depends on how important reserving that data that you have already accumulated inside Marketing Cloud means to your process. We opted for a hard delete of the old person account ID data but that really more boils down to your own company needs.