Unity is really not less than a total 3D environment for developing games .. you can definitely use it to do 2D style games (or 2.5D games, where 3D models are locked on the Z-axis, as you say) but in my opinion if you want flat bitmap-style 2D games you'd be better off getting into something like MOAI (http://getmoai.com/) or Corona or LÖVE2D to get the job done .. (my pref is MOAI right now, its just so fresh and fun..)