I like Kirby and I have bought 2 licenses so far and will probably buy more so I like to pay for it. Just to make that clear first.
Why should I add the license key to the config file? What difference does it make? Does it have some kind of function? If not, why add something that is not used for anything that no one will ever see?
My guess is that a few (well, probably an awful lot of) people need a friendly reminder that Kirby is not free once the site is live. Edit: I presume you created this topic because of 2.1’s license message.
Pasting the license takes 30 seconds, and is no more trouble than what we typically do with other software we buy online (except in app stores).
Adding your license key to the config also makes it very easy to keep track of different license keys for different projects. That way, you’ll always know which license is used in a certain project.
An additional benefit is that clients have the license at hand in case of a problem that needs “pro” support.
And with Kirby 2.1 and higher, adding a license key will remove that message 
By the way, is there any reason ‘localhost’ is not in the list of domains that shouldn’t show the Kirby license alert? I’m asking because I do often use ‘localhost’ locally 
$localhosts = array('::1', '127.0.01', '0.0.0.0');
Great answers! Thanks!