I agree, this sort of stuff is just a cash-in.
(Or providing more grist for the local VC/Angel mill.)
Not all of that happens in 48hours, there's a lot of strategizing that happens before the competition, and a ton of work that happens afterward. BUT, you can create a hell of an MVP in 48 hours if you know what you need to do.
(My point is that you're not charging people $100 for the privilege.)
My main experience with this was a corporate hackathon where a bunch of developers just decided they wanted to build a new product that was different than the one we worked on everyday. That was 7 developers for 24 hours and I'd say we launched a pretty cool product. (http://www.metrodenverapartments.com/ , and about 9 other domains. Looks like the images have broken since.) Also, I was able to demo the product I'm currently working on to potential customers after less than 20 hours of development. At that point the demo was enough that people were willing to commit to paying for it.
It's experiences like this (and seeing others doing the same thing) that make me think these events have value. Here's a quote from http://startnorfolk.com/ (an event my employer and other local businesses are putting on.) Emphasis is mine.
"an intense 48 hour event which focuses on building a web or mobile application which could form the basis of a credible business"
Where is the snake oil in that? My experience is that you really can do something awesome in one weekend if you get together with the right people. It could also form the basis of a credible business. The registration fee is well justified, as it covers food and drinks throughout the weekend. The $10,000 prize is paid for by sponsors.
I attended a StartupWeekend before and didn't feel that anyone thought it was a get rich quick scheme or anything close to it. I personally learned a lot from the weekend, and while the odds are not high, startups have gotten funding through it.
