Sometimes you need a detailed vertical app - like a bug tracker, or an applicant tracking system or a CRM. But a lot of times, you don't. You just need something simple because your needs aren't that complex. And using something you are familiar with is easier than trying to learn something new.
Trello is the always going to be a horizontal app. And sometimes people will need to move to something else to solve their problems. If you are using Trello as an ATS and you start getting 50 applications a day, you are going to go nuts. You need to switch to Jobvite or Greenhopper. If you have a team of 100 devs trying to track your bugs, you want to use FogBugz or Jira or Pivotal.
Joel used a metaphor that I like: The verticals are like stones on a beach and Trello is the sand that fills in all the space between.
In the future I think you'll see the Trello product evolving to support those other verticals and adding value (i.e. quick overview) in the way Trello does best.