1. Less secure than either messenger bag or backpack.
It's not, backpacks and messenger bags are the ones which are the most dangerous in most cases. They move around (most don't strap them) and make you top heavy, which ain't good if you need to make an emergency stop. They are not less secure to what you are carrying (laptop says it's going fine after almost two years of carrying around).
Hassle if you have to stop to go in a store -- do you leave it there and risk it being stolen, or carry it with you? In which case, you've got the hassle of taking it off and the hassle of carrying an awkward burden.
Come on, you just take your pannier and put it on your shoulder (given it's those convertible panniers). Takes about a second to put on and another one to put off. European double panniers are made like shopping bags, and they have a carrying handle. Just google Basil panniers to get an idea of what they look like.
Hassle if you need to take your bike up a passenger elevator (which almost always means vertically).
How so? They take less space than handlebars. Weight is certainly not a factor as you just lock your brakes in the elevator. So far, I haven't had any problem with that.