The BAD thing about a single PIM database, like on the old Palm devices, was if the unified database didn't support something a third-party developer wanted, they had to either have a separate app-specific database (in addition to the main one), OR sneakily encode the data in the existing database in a non-standardized way (e.g., base-64 encoded in a comment field).

Messy. Either way, migration isn't as simple as you might like.