Widgets =/= Apps (or programs).
I think N900 still has "limited multitasking" rather than true multitasking. However, on a screen that small, the N900 can't really multi-task any better and frankly you wouldn't need/expect to do true multitasking.

Nevertheless, the reason why it would be beneficial to have two (or more) programs on the screen are so many in examples that its hard for me to decide on one.

For instance, I might be monitoring a graph of sorts on the right while reading news from a browser on the left.

And I'll re-iterate, this form of multitasking (true) is indirectly linked to the screen real estate, and something like this implemented in an Android or MeeGo tablet would be quite revolutionary to the user (imho).