Keys are time consuming when developing a site. My development process is frequently: clear the wp database, reenable modules, try again, clear, reenable, etc... This has been true trying to use your reference sites. Two possible improvements come to mind:
1) Have a developer key for development sites. One key, multiple sites. Much easier to record somewhere without accessing the toolset.com. The downside of this is that a customer could move their domain away from the developer and keep access to the modules. I guess that would fall on the developer, or perhaps the key would need to be changed annually. I would personally find it useful for all my "test.{domain}" sites.
2) Store a site's key outside the web hierarchy (I do this with aes keys for my drupal development), and allow key access by path stored in the wp-config.php file, or even store the key itself in the wp-config.php file (e.g. TOOLSET_KEY_PATH = '/var/keys/mysite.com/toolsetkey.txt' or TOOLSET_KEY='A1B3dkvi33'). Maybe even a file in the document root, the way google does its site verification (e.g. toolsetkey.html, with content A1B3dkvi33). Any of these would all a developer to set the key once and never have to revisit toolset.com to get it if they need it following a database reset.
You don't need to add a key to a development site.
Toolset works just fine without the registration
On development sites, you can register if you need to use the Installer, but usually, I register only the "real" site.
That is as well the idea behind the whole registry thing, as for example, basic users would not be able to even run a development site, as they can register only one site.
But, since Toolset works fine without it, you can easily run a staging server site and a live site, where you actually register and as well get the support / Updates for
Does that help as an immediate solution?
I guess I should have read the error message -- yes, my development site works just fine without a key. I hate warning messages though, so I may still add it while I'm working. Thanks for the explanation.