Will the add-ons source code be reviewed by a CISSP skilled in the languages used within the add-on? Will the add-on be tested with the top 1000 add-on combinations out there? If the add-on provides an API, will it be tested using fuzzing? The list of these questions, and the others to which your answer is likely 'no', goes on. If you are not doing these things then you are providing a false sense of security. You may catch the bottom 60-80% of malware and unstable add-ons, but the most dangerous 20% will likely slip through, in my opinion.
This does not make sense from a UX perspective, as MANY others here have pointed out, so I won't go into that further. I will point out that it doesn't make sense from a business perspective either. If you are saying your add-on signing program improves security, and you let an add-on through that has malware, then you might be sued (I am not a lawyer, this does not constitute legal advice, etc.).
So to recap and summarize, with brevity, and with accuracy...
EPIC FAIL