Lets assume I have just installed my client. How can I trust the "balances" (I assume you want to store the UTXO set, but details...) when I can not calculate them myself from history data and compare it to the data in the "pruning block"? If I need to do this, someone will have to keep all history data for me to download and verify. Once I verified the data I can discard it. This however is already implemented and its called - you guessed it - pruning.
It removes the need to trust a 3rd party and replaces it with the need to trust the blockchain:
Currently, if you download only a partial chain you must trust a third party for the balance information of the blocks you did not download.
With this, if you download only a partial chain you must trust blockchain technology for the balance information of the blocks you did not download.