Caillebotte turns out to be an important figure in the history of Impressionism. That's partly because of his artwork, which featured humble scenes viewed from unusual perspectives, but also because of his later role as an art patron and collector himself.
Selling eight paintings to buy "Man at His Bath" is controversial for some obvious reasons: Several of the paintings headed to the auction block are by much more famous artists, including Claude Monet, Paul Gauguin, and Camille Pissarro. And those paintings were given to the MFA by generous donors who, perhaps, wanted the museum to have them forever.
Still, selling — or "deaccessioning" — these paintings seems to be the only option. Potential donors, who might not be shy about female nudes, aren't rushing in to help buy a painting showing some random guy's naked butt. "Man at His Bath," another objection goes, isn't a "classical nude" like Michelangelo's "David." But that's sort of the point of Caillebotte's work. Depictions of anonymous people in everyday situations, as opposed to gods and royals and Biblical figures, are a vital part of art.
The lack of eager donors now doesn't undercut the significance of "Man at His Bath," and Caillebotte's work may gain prominence over time as succeeding generations of his family sell off his paintings. Anyway, an art museum — like a music collection — should have aspirations that extend beyond the greatest hits, and selling familiar works to buy an unusual one fits that mission perfectly.