It's definitely useful and a big win. It both clears up general disk I/O, but more importantly (from what I have seen), moves certain key inodes in the delivery file structure off of disk, and since I/O operations have to lock and unlock them for update, the time wasted single-threading through them goes way down
Those I/O locking operations are called synchronous meta-data updates.
Imagine trying to clear an entire stadium filled with millions of people. There is one door. It can open and close very fast, but to be sure that everything is happening safely, it has to be closed and locked between every person. So, you unlock the door, open it, step through, close the door behind you, lock it, and then the next person can come through.
No matter how fast you can open and close and luck and unlock that door, that's going to seriously decrease the throughput you can get through the system.
(this is why, for instance, you should generate a fairly large number of sub-queues in sendmail; if you're trying to do volume and haven't, you're being silly; it spreads the load across more than one inode)
Right, postfix does this by default, and sendmail can easily be configured to do it as well. There are lots of other nice things that postfix does by default and which more modern implementations of sendmail (e.g., 8.12 and above) also do, either by default, or are easily configured to do. In particular, envelope splitting for large numbers of recipients.
This is like having multiple doors that can be operated simultaneously. Even if there is a central operating mechanism that can handle a single unlock/open/close/lock operation, you can interleave the operations between multiple doors, and greatly increase the overall throughput.
Using improved filesystem architectures, such as softupdates, would be like using revolving doors instead.
Using a RAM disk instead of physical drives is like making those doors operate at light speed, instead of the limitations imposed on normal wood, glass, and steel by the laws of physics.
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