If you are talking from taxation perspective (as you have mentioned in your subject), then yes MO Operating Unit profile option is required for GL responsibility if you wish to calculate taxes at GL level. The tax codes are derived based on this profile.
MO Security Profile is defined for the GL responsibility if you wish to control the security on drill down. But then you also have to enable the profile option "SLA: Enable Subledger Transaction Security". If the security is not enabled, then you can drill down to any subledger transaction. If the security is enabled then you can only drill down to those operating units that are part of the security profile attached.