Mental growth and pain, errr determination is the reason I run single speed(sometimes). Going up a hill, you have to do whatever you can. Build momentum, spin+pull, stand, shift muscle groups, etc. Of course you can do all of these things on a geared bike. That's not the point, it's to lock in your options of walking up the hill, or aggressively attacking it so you can get up it. Since I was riding single speed for most of my childhood I am just used to it. As others have said you can try it out on a geared bike before you make the decision.
I also have a single speed MTB and that one has an easy setup that locks me into zone 2 since I tend not to spin up like crazy on the flats.
I found with brifters I used to shift gears too much, and with DT levers that's a non issue, but I am weird since I ride alone and am fine with wide spaces in between my cogs.