First, a ledger system provides no meaningful resources savings (no reduction in storage space, no reduction in memory usage, no reduction in network traffic, no reduction in CPU usage) because we are already very lean, meaning that our transactions are very close to the absolute minimum amount of information necessary.
This is not true. 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=195275.0Furthermore there is always only single entry for each address while bitcoin can hold any amount of unspent outputs for single address.
Transactions in such system are much smaller, because you don't even have to include pubkey hashes but just offsets of account in db. I made some calculations and tx could be as small as 24 bytes + signature.
Second, a ledger system is necessarily less flexible and useful. None of Mike's contracts would work in a ledger system without adding trusted third parties. You could regain that functionality by emulating the current scripting system with the scripts attached to disposable accounts rather than disposable transactions, but why bother? Bitcoin puts the complexity where it belongs, in the transaction.
Well bitcoin is more flexible only in theory. Most functionality of scripts is turned off until developers decide such scripts are nessesary and won't bloat blockchain, network etc. It's effectively not more flexible than account ledger.
Bitcoin scripts are constrained while with ledger you can make more powerful concepts because you have access to turing complete programming language and current network state. You can make complex constructs for example to make secure atomic laundry (
https://bitcointalk.org/index.php?topic=195275.msg2289285#msg2289285) and much more.