That's true of the taskbar in Windows 7 too, but in practice people don't tend to have bajillions of applications or WSapps open at a time, so it's not a big deal.
Generally you only want a complete list of apps if you've lost something you were using ages ago (in Metro you get this by clicking the app again from the start screen) or to close down the apps you're not using (you can do this via the new easier-for-noobs-to-use Win8 task-manager or by right-clicking the items in the Metro-taskbar to close).
Apart from those two things which are both catered for in Win8, what else do you need the Metro-taskbar to do that it doesn't currently?
It's not true of the W7 taskbar; it can be multi-row, and can contain a ton more apps.
And if you follow the idea that people don't close their Windows Store apps then they will have a bazillion apps open. Ok maybe an even dozen.
The task manager is fantastic but not something that Joe Schmo is going to know how to get to easily. Oh wait that's everything in W8 so you're right. They will find it useful.
The Windows Store app taskbar is more like a MRU list and not like the W7 taskbar that is so uber.
What else should it do?
Show a nice "x" on the thumbnails that I can click to close the app (like Windows Store IE's tabs) without having to right-click to bring up a context menu.