Including a famous person as coauthor?

Let's say a famous researcher contributed only a bit to a research project by a student, and the student could reasonably choose either to include or not include them as a coauthor.

The student doesn't get to choose, the student gets to invite: It's the famous researcher's decision as to whether they want to be a co-author, since they contributed a bit to [the] research project.

It seems that having a big name would increase viewership and perceived legitimacy of the paper, while having more coauthors also seems to diffuse the ownership of the work.

The benefits are significant, whilst the drawback of more authors is minimal (albeit, fields vary).

Yes, it is beneficial.

See the paper "Early coauthorship with top scientists predicts success in academic careers" in Nature Communications from Nov. 2019 (or the short news version at Nature Index).

In an analysis covering more than 22.000 scientists, it finds that collaborating with a top scientist early in one's career leads to an almost doubled increase in the probability of becoming a top scientist oneself.