The main advantage is that not everything needs to be in place in one go (at the transition height). Noticeable improvements can be tested in production, and experienced by users along the way.
Note that this is purely a thought experiment (called "Dothereum"), so there's no plan to implement it. However, we develop and provide the rough specifications as inspirations for both #Polkadot and #Ethereum ecosystems.
Dothereum is a thought experiment descirbing what eth1 -> eth2 transition...GitHub