Freedom and Transparency

This is a good reason for opting for your personal e-mail account instead of your institutional one.

There have been several efforts recently by partisan groups in Michigan and Wisconsin to gain access to faculty email messages on subjects that fall within the scope of the faculty member's research or personal political opinions. These groups have made use of state Freedom of Information laws, on the basis that faculty members are "state officials" and their communications are therefore "business records" of the university.

Source: UnderstandingSociety: Academic freedom and faculty email