I can see the value of that restriction as an optional security measure, but there are many projects that use outside contractors or even open source projects with contributors from all over which would need more openness than this.
Makes sense, but when someone joins the team as a contractor, s/he will have to have the company's email anyway, right? For example to be included in the email distribution lists and to be visible in the company's contacts.
I am interested to get feedback from the community members. Do you have contractors that work as part of your team, and do not have a company email?