It depends on the definition of 'at the same time', but I think that many people in a work environment at the very least use an e-mail client and a productivity application at the same time. Also throw in a calendar for good measure.

I think that Apple thought that many regular users would switch to full screen apps on the Mac, combined with Launchpad (it's just like an iPad/iPhone). But virtually all non-tech-savvy Mac users that I know do not use Launchpad, nor fullscreen apps.

I think the problem with Launchpad as with Spotlight search [1] is that they are not very discoverable on the Mac. Having search in an application menu (like recent Windows versions and some Linux desktops) is far more discoverable.

I guess people don't use fullscreen apps because they equate desktops/laptops to the 'WIMP' interface paradigm.

[1] If I received a penny every time I see even experienced Mac users launch applications by clicking on a Dock icon or by navigating to the Applications folder in Finder, rather than using Spotlight, I would be rich.