What do you mean by multitask?
Actually doing more than one thing at a time (not just allowing you to swap between tasks but have background tasks dormant).
That's a fallacy. No one works in more than one app at the same time. If you'd said background task (i.e. kick something off, and go into another program while that's still processing) I'd agree with you; though some apps can be backgrounded and still work, others can't, and it's something that I do find limiting in some cases. But other than for reference purposes, or sharing information by drag/drop, no one actually works in more than one program at the same time- the mouse related interface that we have on computers doesn't allow it. I hope in the future multi-touch does allow this (a la Minority Report), but for now, it's an illusion that's been propagated, and doesn't survive the light of reason.
For the most part, I think you're right there.
But... (you knew that was coming)... If you have say a Word document and a PDF open, and you type in the Word document while reading the PDF, are you "using more than one app at the same time"?
A for multi-touch as in the Minority Report, isn't that all just one big app?
I think that by our nature, we are incapable of proactively using 2 apps at the same time. Passive? Sure. Just like the PDF above. We all use LOTS of them, and even ones we don't know about. Drivers, daemons, services, agents, etc. etc. But proactively? That requires multiple input devices/sources.
If you had a sensor to detect your heartbeat, and that was hooked into an app, that would still be passive.
If you are typing or clicking with a mouse or speaking into a speech recognition powered app, that's clearly proactive.
Let's take Star Trek for an example. You're the ship's navigator and you're all alone on the deserted ship... cue spooky music please... You're frantically typing/tapping things into the computer console to avoid some Klingons pursing you... All the while you're shouting commands to the ship's computer...
Are your voice commands active? Well, yes. But how much concentration do they take? Is that using multiple applications? Or is it using one interface (the console) and intermittently actively using another interface?
I think our powers of concentration are limited, and that is the absolute limiting factor. i.e. How much concentration does an application require to use as a percentage of the users available, productive attention/concentration amount?
I think that at the end of the day, we're mostly just quickly switching between applications and that we're never really using multiple applications, no matter what the interface. (Unless it's cybernetic, but then it's still passive...)
Kind of like how mobile phones are illegal to use while driving. People don't have enough concentration/attention to do both reliably.