Hy, I just found useful to post my idea here.
I've seen videos about automated quadcopters: http://www.ted.com/talks/raffaello_d_andrea_the_astounding_athletic_power_of_quadcopters.html and http://www.ted.com/talks/vijay_kumar_robots_that_fly_and_cooperate.html.
I surfed pages from the companies presenting this research and other information on the internet, but I haven't found why they use quadcopters specifically. I understand, how accelerating, rotating and rolling works in those quadcopters - it's simple, but they claim that quadcopters have minimum number of working parts to fulfill their needs, which I don't agree and I think that tricopters are better in this (duocopters can't rotate horizontally, but tricopters can by inclining and then powering the remaining left or right propeller).
I rode forums, calculated and draw drafts of both tri and quad and found, that tri is much more efficient just in everything than quad with same props and battery when taken in account that best properties has the smallest copter with the largest props so: 3:4 moving parts (no vectored yaw in tri), 9.5:16 size, building Y instead of X construction take far less material 1.5:2.82, lesser maximum power input 3:4, better power efficiency makes longer flight time and tricopters have also improved agility over quadcopters.
The only disadvantage I see is a bit complicated horizontal rotating in tricopter without vectored yaw, which can be problem in man controlled machines but easily solved by simple algorithms in automated machines -> it's not a real disadvantage, just a small work to be done. I was thinking about doing that in my bachelor thesis, but for now I am looking for your opinions, thanks!
EDIT: Maybe the torque is the problem, because on tricopters you can can have all 3 props in 1 direction or 2 in 1 direction and 1 in the opposite and it's symmetrical in neither way, but I'm not sure if this is the main problem...