Surely the "coins" stored in wallet.dat are essentially just a cache, though, right?  The "real" count of BTC that I can spend are encoded into the chain.

If I still have all my private keys, I should be able to scan the block list to recover that "cache".

Suggestion: Include a block number/id in the wallet which is the last-known-good point for that wallet.  When a wallet is restored from backup, you can rescan the new blocks to be sure it's up-to-date.

Second Suggestion: All the user to rebuild a wallet without re-downloading all the keys.

Final Suggestion: Separate the "real" critical data (the private keys), from the "cache" (my current coins).  There is no reason to conflate the data that is absolutely critical for me to access my coins from a system designed to efficiently find my coins and let me spend them.

(Unless I'm fundamentally misunderstanding the inner workings of BitCoin!)