Sadly, the core client has the job of being capable of bootstrapping itself starting at the genesis block and ending at the most recent block.

That means that, barring a huge design change in Bitcoin itself, the core client has to support every single feature ever implemented in Bitcoin, no matter how half-assed or poorly thought out.

That's why things like the way P2SH got implemented really grind my gears. Bitcoin has a few idiosyncratic and unintuitive design additions that make it really hard to make simple clients.