If "you still don't get it" sounds condescending, which I honestly don't think it does, then I apologize, but I am not trying to be superior. I am trying to explain to you how something works, which you are assuming it doesn't.
When an app goes from the foreground state to background, of course... it can directly influence the amount of memory free since it was the last application to have priority over memory usage. The next factor is how much memory the app that is being switched to the foreground will ask for. So if the background app freed up enough memory for the next app to launch without asking for more, the request will be granted. Unless you are trying to edit a photo while running a 3D game, it's not likely that a low memory condition will occur and block the task completion request. This will only get better as iOS gets more memory to play with.
Next is... none of the current apps on either iOS or Honeycomb will feature such "heavy" tasks as you have noted. There is no 3D modeling app for Honeycomb. There is no photo editing app for Honeycomb. There is no movie editing app for Honeycomb. There is no music studio app for Honeycomb. There is just... nothing that will do tasks heavier than downloading/uploading stuffs or applying image filters on these devices. It's just not happening.
So right now, and I mean... no theory, no speculation, no projection, just fact, and fact is... iOS multitasking does things just about on par with Honeycomb, and there are more things you can do on iOS than on Honeycomb. Add iMovie and GarageBand to the mix and it's easy to see how much more iPad 2 can do than the XOOM, multitasking or not.
Lack of software stability is not the issue with the XOOM. It just simply... lacks everything related to software. If you want to go into productivity, then there are more apps that can do a lot more on iOS than on Honeycomb, even if you can't have them run automated tasks in the background. A pretty interface, widgets, and convenient multitasking don't help much when there is nothing else that can help you do your tasks on the device.