Grandstanding and networking sound like b-school entrepreneurship competitions. Who can make the best slide deck. Plenty of companies got funding with nothing more than a few presentation slides and some good talk so I agree it's a potentially valuable thing. It just doesn't reward technical prowess so why feature it at all?
Hackathons are a blend of all of this and it doesn't really do any of them well.
My main complaint about hackathons is that they are a poorer recruiting tool than most people want to believe. Corporate sponsors think they'll be able to spot top talent easier. And some participants think hackathons improve their chances of getting hired. In my brief experience, the success rate is very low on both sides. At best, I think a hackathon is a fun, creative, and social collegiate experience for the participants and another avenue for companies to market their brands to impressionable young minds.