The M³WAAG DKIM Key Rotation Best Practices document (pdf) recommends a "sufficiently" random DKIM selector name so that it cannot be guessed by browsing the DNS. A literal quotation:
4.3 Key Selector Naming Scheme
Define a naming scheme for the DKIM key selectors that is both meaningful for forensic analysis and is sufficiently random so the keys cannot be easily guessed by browsing the DNS.
NOTE: The selector naming scheme should also be designed to mitigate the risk that attackers can easily predict the names of future selectors and retrieve the associated keys. See Section 5 for a description of the process for publishing keys for future use
This may be relevant for short 512-bit RSA keys, but it does not seem to make sense to me for longer, say 2048-bit, RSA keys. The DNS holds public keys which are not secret and can be discovered by reading just a single signed mail. Security by obscurity with very little security?
Why would a random DKIM selector name be better, when would it make sense to follow their recommendation?