The best way to measure the value of Wikimania IMO is through stories. So here are a few of mine:
1) In Washington, DC a group of us meet regarding a new sister site. Partly from this, two editing communities working on travel content were brought back together and Wikivoyage was reborn as a WMF site.
A gentleman from the World Health Organization flew in to meet with some medical editors. A six month Wikipedian in Residence at WHO followed, I was invited to speak in Geneva, and the Bulletin of the World Health Organization is now under a CC BY SA license with work for them to more fully adopt an open license.
2) In Hong Kong I meet with the gentleman who runs Healthphone http://www.healthphone.org/ He has subsequently agree to release some amazing pubic health videos under an open license one of which you can see here on Hindi Wikipedia https://hi.wikipedia.org/wiki/%E0%A4%B8%E0%A5%8D%E0%A4%A4%E0%A4%A8%E0%A4%AA%... Discussions are ongoing to have WP's medical content included on 2 million SD cards being shipped to healthcare workers in India.
3) In London I was introduced to volunteer programmer through a friend of a friend. We had an idea for a copy and paste detection bot and we just needed someone who knew how to code. Eran did the coding during the conference. The bot is now live on all of En WP with other languages in the works https://en.wikipedia.org/wiki/User:EranBot/Copyright/rc
4) In Mexico we continued to improve the copyright bot and pulling in a few more programmers. Our community liaison for the project got to meet community members face to face for the first time. We also solved some issues regarding maps that had the potential to go sideways.
I am certain many other have similar stories. I have personally found Wikimania invaluable. Attempting to bring us together once a year is not too much.