That could be interesting, but then what do they need you for? I can already do that with gmail. I just set up a domain on google apps and then tell it to deliver any email address to my main email. In fact I do that already (see my profile).

The convenience factor is definitely there. A browser plugin that automatically creates an email address that you can later turn on/off with a couple clicks instead of going through the domain setup in gmail (or in my case fastmail)? I'd be interested in it.

Price, convenience. Google Apps is not free. For this particular use case you might be able to provide better user experience. Maybe integrate the custom domain registration and automatic DNS config to the UI.