I sort of get it but don't really. Separate from battery life/heat, I don't see any downside to multi-tasking on a phone but, with the limited screen real estate and what multi-tasking truly means, I don't see much upside either.
If multi-tasking is simply returning to where you were, I don't get it at all.
If multi-tasking is notification, I think notification is the important point.
I think multi-tasking is really only useful in the phone context for non-visual applications that require being always-on. For example, listening to music, beaming location information, etc. And note that beaming location information can probably be handled through a subset of notification (rather than flashing a box with the notice to the user, have the phone respond when pinged from the outside).
At the end of the day, I don't really "multi-task" on my computer either - I work on one application at a time and only keep more than one open simultaneously because I have the screen real estate and start up of each program is slow.