CiviEvent is one of the more popular components in CiviCRM (followed by CiviContribute, CiviMember and CiviMail). We have improved CiviEvent significantly in the past few releases and more improvements are slated for CiviEvent in v2.1. With this increased usage, we've seen more folks using Price Sets. Briefly, price sets gives admins the option to break an event into smaller pieces and charge for each selected piece. Price Sets was one of the first significant of code contributions to CiviCRM (thanx to Ideal Solutions and Marshal Newrock). We were planning to restructure this code in 2.2, but there were two missing features which came up a few times with some of our 'regular' users.
The first missing feature was that two options in a price set item could not have the same amount. In order to include meals on multiple days with the same price, you would have to separate them into different items. Price Sets did not allow an amount of 0 either in an option. Fixing this was relatively easy, we just shuffled around a few fields that we used in the civicrm_option_value table to fix the above problems.