Reasons to include a dislike button?

My company is making an online search engine for teaching resources. The site indexes resources from various educational sites (TES, BBC, etc) and displays them in a post-list format. The site will primarily be used by teachers, who are short on time and not necessarily technically minded.

Example resource:

We want to give users a way to rate the resources. I have opted for a like/dislike system rather than star-ratings for reasons such as this and this. Users have to login to vote, and can only vote on a resource once, to prevent (or at least reduce) spamming.

One of my colleagues believes we shouldn't have a dislike button. He believes it would create a "negative feel" on the site, and source websites "would not be happy with their resources being disliked".

I disagree, because I believe our site fulfills the criteria in this answer. What do you guys think? Can I get further opinions/sources on whether a dislike button is suitable?

EDIT:

I'm ideally looking for articles/studies that support my point that the term "dislike" is suitable here.