Might want to read further in the Mac forum thread, esp Bosco and Vpndevs post on that forum
http://www.macobserver.com/tmo/answers/4-wi-fi-tips-from-former-apple-wi-fi-engineer ... Also there are many non-apple devices that don't quite handle the band switch well or at all and leave you to choose the band (Roku anyone?)
Its a interoperability issue.. Lars explained some of it, along with the other forum members (Zimmie) In an ideal world.. this would work... but also realize that Apple networking equipment doesn't always play well with others.. Like everything with a certification, theres still room for interpretation and optional components... (mDNS issues anyone?) ... Might want to read some of the posts here and on
http://www.smallnetbuilder.com/forums/showthread.php?t=15656 (which is an Asus forum)..
Your apple device might autoswitch, but many other devices just pick the stronger signal.... By having the two frequencies separate, you can make the choice for yourself and not rely on autoswitching.. And frankly IOS 7/8 have issues with autoswitch.. Known issue with IOS 8 btw...Youll save headaches esp with non-apple devices on your network by making the choice yourself than relying on autoswitching from 5 to 2.4g..
This reminds me of a JTS harddrive I bought from a whitebox seller ... It was to one of the higher speed IDE/ATA specs.. it had compatibility issues however... most of the drives were returned.. Problem is.. even though it was to the published specifications, there were things that this newer company did not know were updated/undocumented revisions from the published spec, but were known by the major players in the industry.. and weren't shared....
Or you might have a device that sees both 2.4 and 5.. but the 5 is an unstable connection (due to distance or interference factors).. and it wont switch to 2.4... Have had this problem with streaming devices (Roku and a few others. Dont let you even see the diff between 2.4 and 5 if same SSID and of course they dont want to fix it.. But its not a single instance, just an example)
Apple networking devices hide a lot of configuration from the user, at the cost of not playing completely well with others..