[akira2501]

I believe you would use it to determine a random outgoing port to use to contact the DNS server; this prevents spoofing. However, the port space is only 16-bits, so how you map the outputs of the MT into that space would have the biggest impact -- but you're right that's it's probably best to avoid it entirely.

[mike-cardwell]

You also use it for randomising the 16 bit ID in the request packet, and you can use it for randomly capitalizing letters in the qname (0x20 bit encoding). Both of these to help protect against spoofing.