Ledger system done right provide big savings in memory, because you don't even have to store old past transactions. You can discard it after they are buried under enough blocks. See this for details
https://bitcointalk.org/index.php?topic=195275.0Using balances is a very good idea, much better than current BTC.
But deleting old transfers is not GAAP-conform, therefore a bad idea.
See above about closing books and carry-overs: We start a small new database, but keep all previous transfers available in archives.
https://bitcointalk.org/index.php?topic=211835.msg2307474#msg2307474Any BTC user should be able to read and download the archives.
Legally financial books must be kept available for at least 10 years.
Anyone can keep archived transactions for how long he want (and it's validity can be checked by anyone). I think especially node should keep all history of its own addresses. Full history is just not necessary for payment processing so standard nodes would just drop it and save history logging for specialized nodes.