For me, moving my hands from the drops or hoods to the top of the stem to shift is a very unnatural motion, requires me to move a lot more, and I sometimes even find it hard to hold my line while doing it. Reaching down to the bar ends or down tube to shift feels much more natural. YMMV and this could very much be a matter of what I'm used to.

If you've never used them, I highly recommend picking up a set of barends and giving them a shot.