Wow thought with that many apps going there would be a performance issue which goes to show you how wonderful having all that RAM in the iPad 3 does for it.
The iPad automatically closes older apps when it needs more RAM. The number of apps in the list means absolutely nothing, other than those apps were running at some time in the past, the most recent first on the list. The iPad 3 (by my brief tests) can hold about 20 moderate sized apps in RAM.
Normally, only the current app is running. All other apps are frozen in RAM and use almost no CPU. There are exceptions, but those exceptions are limited to the few types of background tasks that iOS allows, not the entire app running and using iPad resources.
That is why the iPad remains responsive.
A misbehaving app can change that, and is the primary reason for manually clearing an app from RAM (deleting it from the multitask bar). It forces the app to reload from scratch.
The other reason is that no operating system is perfect, including the RAM clean up routines. Sometimes clearing out all the RAM can help when launching a large resource hungry app, especially on the iPad 1. Restarting would do the same thing, but leaves the list intact, which tends to confuse people. If nothing else, it leaves them wondering what is or isn't in RAM. If you clean the list regularly you have at least a rough idea of what
might be in RAM.
If you'd like a more detailed explanation, this is my favorite; at least for now.
Fraser Speirs - Blog - Misconceptions About iOS*Multitasking
There are more caveats and exceptions than the article mentions, and there are certainly people who feel the article is wrong in intent; mostly because they feel that iOS doesn't doesn't do a good job of cleaning up RAM, not because it isn't supposed to work the way Speirs outlines in his article.