When you have 2 tanking classes that are balanced, and are in the same raid, no one straight puts one of the offtanks there because it's somehow slight inferior. There are many factors for assigning offtanks. Connection, gear, skill are all taking into consideration along with the actual duties required of the Main Tank and Offtank. An offtank for one boss could be main tank on the next. Asking to always be maintank is selfish and is how unbalanced things are right now.