I think they are a great way to have a break from regular development. I love 'em

I worked for a company which runs one a year, and I honestly look forward to it. Having a chance to cut loose and show what you can do is a welcome change from the daily grind.

It also lets you show stuff that honestly you are more interested in, and maybe, you can get a product out of it - which means, you get to work on what you actually enjoy. As a way to influence a business to show what is possible, there isn't a lot better.

I think they have their place. Just not as a BAU thing.