It had quite a large amount of staff members compared to a normal ship and there are actually quite a few Commanders on the ship who weren't members of the regular cast. Remember the Commander in charge of the Astronomy Department that Picard fell in love with? Based upon the command structure, I think we can assume that every department had a Commander or Lieutenant Commander as the departmental head (especially in the Science and Engineering areas), and the only reason Troi (she's one of my favorite characters, so this is not a bash against her) was so visible was due to the diplomatic missions the Enterprise was often sent on. If the Enterprise was a full-scale Science or Medical vessel (and if Star Trek were created with an unlimited budget for casting and special effects !), we'd probably see more Commanders heading up more obscure specialized departments (ie: Molecular Biology, Flight Software, R&D, Advanced Tech, etc etc).