To open source or not to open source

Yahoo allows outsiders to innovate on Yahoo e-mail

Yahoo has decided to open the underlying code of yahoo mail to outside programmers. Now this can be a good thing and a bad thing. Of course we will see a lot more applications built on top of yahoo mail, but then it is a also a nightmare from the security point of view. On one side, since the source code is allowed access they are more vulnerable to attacks. On the other hand, how secure will be the newer applications which are going to be integrated with yahoo mail.