I am quite accomplished with, and comfortable using, the trackpad on my MBP. Despite that, on occasion (particularly when I've recently been using my iPad), I find myself putting my finger on the screen to select/move/change something, before realizing that doesn't work.
A touchscreen on a laptop is absolutely not useless. If Apple made the MBP screen an iPhone-caliber touchscreen while keeping the multitouch trackpad, I would certainly not complain. I'd use it along with the trackpad and keyboard. For some things it would be quite useful.