Desktop touch functionality wouldn't necessarily be added to the primary display. Apple has filed patents for using touch screens for keyboards and trackpads. Even if it was for the primary display, apple has a patent for a movable display that is oriented horizontally when used as a touch screen.

It's not clear how or whether would add desktop touch features, but they've clearly considered it.

Touching your display is a horrible idea (I wonder how those new windows machines that do this are selling) [1], but the trackpad on the MBP is the thing that finally made me buy one.

[1] Imagine working whole day having to hold your arms up to operate the mouse cursor. My arm hurts just thinking about it.

Sometimes it is useful, very useful, most time it isn't. This does not mean it is a bad idea.