Reputation is built with journals, not publishers.

Publishers _do_ add value to _conferences_ by helping out with funding. Since most smaller interesting conferences aren't at all profitable, it's a "symbiotic" relationship between the conference and the publisher: the publisher effectively runs the conference in return for having exclusive distribution rights over the proceedings. Sucks for the profs and students though.

When a conference gets big enough to raise their own funding _and_ bold enough to shirk their publisher's exclusivity contract, in-house distribution begins to make sense.