A blog post prior to tickets going on sale explained changes to the show this year, including the reasoning of only offering single day passes:
Change #3: Lastly we will be doing away with 4-Day badges. When looking over the data, we saw that people were buying 4-Day badges and not using them for the full four days. By switching everything over to single days, we can help ensure that every sold badge actually gets used. Since the show isn’t falling on Labor Day, it would have made the “not using the Monday part of their 4-Day badge” situation even more pronounced.
Hopefully with all these changes, we’ll be able to provide an especially enjoyable environment for everyone and make sure the maximum number of people can take part in the festivities.
Personally, I don’t see a problem with the folks behind PAX deciding to institute a single day pass format; San Diego Comic Con did away with multiple day passes last year. I do understand some of the frustration though since I’m sure fans would have appreciated knowing the PAX decision before they logged in to buy tickets…
