Currently NEP-21 defines account->balance map as:
#[near_bindgen]
#[derive(BorshDeserialize, BorshSerialize, PanicOnDefault)]
pub struct FungibleToken {
/// sha256(AccountID) -> Account details.
pub accounts: LookupMap<Vec<u8>, Account>,
Using hashes is problematic for reverse look-up reasons. For example, if I want to print out a human-readable CSV file for all token holder addresses. With current NEAR infrastructure in-place, this would be difficult. Events will probably makes this much easier.
Probably nothing needs to be done for this now, but this should be addresses in the future token standard examples, so that developers have an answer to the question “how to print out all token holders”.