If an attacker needs to guess a key, there is nothing to worry about. The keyspace is way too large for that.

If an attacker has access to your wallet/backup/passphrase/... in a way that grants him access to one of the keys, he very likely has access to all keys.

There is one small security difference between deterministic and randomly-generated wallet keys: if someone manages to copy the keys from the second, he cannot wait (long) before stealing, as the coins tend to move to newer addresses (i.e., it becomes "unstolen" over time).

Also, there are plans to implement deterministic wallets for the reference client too, as the advantages for backup safety far outweigh the security risks.