1. Its not simply about they not being able to "do anything" to my account. Its the linkage. Maybe I use my real name on twitter, and I do not wish to use it wi

1. Its not simply about they not being able to "do anything" to my account. Its the linkage. Maybe I use my real name on twitter, and I do not wish to use it with the service I am trying out (or the otherway round). The linkage extends even further. If I signin to both, service A and service B with the same twitter login, now these two can link to each other, without any assistance. 2. It takes mental effort to keep track of all the permission I may have granted to various services from my various identity providers. If I want to disable access, I need to make extra effort. If there is no linkage, I merely forget it about it and it goes away. Even you say "They don't get your email (I think)" - that "I think" right there is the problem. I would have to keep (and maintain) a mental checklist of everything I am providing access to when I use a syndicated login service. 3. Permissions evolve. Twitter may change its access policies, a new hack might allow privilege escalation. Each time this happens, I would need to review what apps have which permissions. For services I actively use, I am willing to put in that effort If absolutely needed, but for something I tried out as an evaluation, its not worth the hassle. As I said, until I am convinced I am going to be using the service, I do not wish it to stick around in my "apps with permissions" list, or anything else for that matter. If I try it out, and decide it aint for me, I just want to navigate away and never think about it again.