Leak shows HTC M8's on screen navigation buttons
It seems to be a personal preference. Some like the on-screen buttons, while others would rather have the capacitive keys. The advantage in using on-screen buttons for larger-screened devices is that it can reduce the actual size of the handset. And on-screen buttons also might lead to fewer accidental touches of the navigation keys.
A published report on Sunday revealed an image of the alleged HTC M8 online along with the capacitive buttons used for the HTC One and HTC One X. You can see that if the image is legit, HTC will be returning to a three button layout for the HTC One 2, even if it is on the screen. Last year, HTC's navigation panel featured just the home and back buttons.
source: Weibo (registration required), HTC Source via AndroidandMe