As explained in the Design Rationale wiki:
Using sha3(k) as the key in the "secure tree" (used in the state and account storage tries): this makes it much more difficult to DoS the trie by setting up maximally unfavorable chains of diverge nodes 64 levels deep and repeatedly calling SLOAD and SSTORE on them. Note that this makes it more difficult to enumerate the tree; if you want to have enumeration capability in your client, the simplest approach is to maintain a database mapping sha3(k) -> k.
Does any client maintain such a mapping? It'd make it possible to enumerate all addresses in the network easily (see this answer for context How to list accounts / addresses?)