I'm still trying to figure out the use for this. You have to activate it by pushing something or swiping something, and then swiping through the list of open apps, and then click on it. Isn't that the same exact steps you would go through to actually open a normal app? Just sayin...
It may seem that way, but let's say you have an app open. You can double tap the status bar to background that app (which also opens the PS interface), then swipe to another backgrounded app, and go into that one. So essentially, you can switch between "X" number of backgrounded apps without ever returning to the springboard. It's much like Kirakae in a sense, but IMO has an incredible interface, much like the Palm Pre "card system".
edit:
Here's a decent article describing ProSwitcher
http://www.macstories.net/iphone/proswitcher-beta-multifl0w-update/