With all due respect six, your view of Android is seriously flawed. You need to check out newer devices and Gingerbread.
multitasking allows apps to use CPU cycles in the background, instead of just holding them in memory. For comparison, it's like if you were to minimize your browser with a YouTube video playing, and your computer just froze it in memory (video stops, browser uses no CPU cycles), until you came back to it. It's good for some things (battery life, maybe saving some memory) but many services and other features which are very useful require it. It's kind of a Catch 22.
It is strange that my Samsung Omnia from 3 years ago (Windows Mobile 5.1, yuck) had true multitasking, and iOS still doesn't...
EDIT: Also as magus said, they release some of the cache to save memory
EDIT 2: Here's a good reference http://www.pcworld.com/article/19983...for_users.html