When should you specify PAD_INDEX?
From Kimberly Tripp's post:


  Typically, PAD_INDEX is not needed unless you have extremely unbalanced activity and are unable to rebalance the tree (meaning you need to achieve high availability).


Read more: http://www.sqlskills.com/BLOGS/KIMBERLY/post/MSDN-Webcast-QA-Index-Defrag-Best-Practices-Fragmentation-Questions-Part-I.aspx

What she means by high availability is that you don't have the ability to run index maintenance jobs that would lock the table.  Personally, I wouldn't recommend ever setting this by default - leave it to DBAs who find that they're unable to manage index maintenance appropriately with any other methods, like using Ola Hallengren's maintenance scripts during off hours.  His scripts are genius - they'll do as much as possible online.