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.