Advantages to taking this approach are numerous.
- OpenPGP is a well known and tested standard
- Security is built in
- There is room for growth and modifications
- It's very robust and covers almost all use cases
- Possible use of existing OpenPGP compliant architecture
There is one great disadvantage, though. With robustness comes complexity - it's not a quick and easy fix to the problem of key sharing but rather a lengthy process of integrating bitcoin into OpenPGP ecosystem.