Performance. It should be analyzed from case to case. I think that keeping all toghether in one tablespace becomes another myth too! It should be enough spindles, enough luns and take care of queuing in operating system. if someone thinks that making one tablespace is enough and is the same like many tablespaces without taking in consideration all other factors, means again another myth. It depends!
High Avalilability. using separate tablespaces can improve high availability of the system in case that some file corrution, files system corruption, block corruption. If the problem occures only at index tablespace there is achance to do the recovery online and our application still beeing available to the customer. see also: http://richardfoote.wordpress.com/2008/05/02/indexes-in-their-own-tablespace-recoverability-advantages-get-back/
using separate tablespaces for indexes, data, blobs, clobs, eventually some individual tables can be important for the manageability and costs. We can use our storage system to store our blobs, clobs, eventually archive to a different layer of storage with different quality of service