Actually, roman numerals are easier to sum than arabics. To sum two roman numerals you just write all the digits together and collapse them as much as possible. There is no carrying over of digits and such.
Otoh, arabic numerals are a lot easier to multiply/divide. Before the wide adoption of arabic numerals all but the most educated couldn't perform simple multiplication/division.
You may have a point, but I still find easier to do
111
948 948
289 => 289
--- ---
1237
CMXLVIII plus CCLXXXIX => CMCCXLLXXXIXVIII => MCLLXXIXVIII => MCCXXXVII
but it may be that it's because what I'm used to.
Are you saying that Roman numerals are better for counting than Arabic numerals? I suppose you don't mean count (I, II, III, IIII, V...), but show the number of men in the army. I don't see any advantage in using Roman numerals.
Lets say we divide infantry in XXX maniples of CXX men each, supposing they are complete and you haven't lost people yet in battle. How is that better than 30 maniples of 120 men?