I was recently invited to speak at two conferences. Taking into account the time it takes to prepare a talk, to travel to the venue and to attend the meeting, it is a very inefficient means of knowledge exchange. This is why I only accept invitations if my travel expenses are reimbursed and if the conference is held in a place I haven't visited or would like to visit again or if I'd like to meet some of the other invited speakers.
Because of course, the thing about academic conferences is that the secondary goal, meeting colleagues and exchanging gossip is primary and the primary goal, presenting your findings and ideas, is secondary.
I remember reading a story about the curator Hans Ulrich Obrist who once invited a number of speakers to attend a symposium, which at the very last moment, when everybody had already arrived, was cancelled. Everybody loved it, because people didn't have to worry about their presentation and could spend all of their time discussing with colleagues. This, I think, is what a symposium should be like: an opportunity to meet and talk with colleagues from the same or different disciplines in an informal setting.
Some conferences are so huge with so many parallel sessions that they inhibit more than they advance the exchange of ideas. I think the number of attendants at the annual meeting of the Society of Neuroscience exceeds 30,000. It's the same in physics and other areas of medicine.