You choose to view the architectural model as you please and where you please. You can even host local MEP components (fixtures) on linked model objects such as walls, ceilings and floors. But there is an issue that autodesk has to address......read on....
One current issue with REVIT MEP seems to be with regards to 'surface hosted' vs 'object hosted' families. REVIT MEP components such as electrical and lighting fixtures are created (out of the box) as 'wall hosted','ceiling hosted', 'floor hosted' etc.
Do you want all your fixtures to dissappear if the architects wall or ceiling dissappears? Maybe or maybe not. I'd say leave it up to me as to what I want to do with orphaned fixtures.
Now, do we start to convert all the 'object hosted' components to 'surface hosted' components or do we wait for autodesk to solve this fundamental problem? I would say just convert the components we use the most and wait to see.
Below are some Autodesk produced videos giving an overview of a few REVIT MEP topics.
(My apologies for the dead autodesk links below. I will get them resolved)
Electrical Overview Videos
Electrical Lighting and Power Circuitry
Electrical Lighting Calculations
Electrical Power Load Balancing, Wire Sizing and Panel Schedules
Electrical Wire Tags and Lighting Switch Topology
Fire Protection Overview Videos
Fire Protection Sprinkler Placement
Heating and Cooling Overview Videos
Heating and Cooling Load Analysis
Mechanical Overview Videos
Mechanical Design Criteria
Mechanical Duct Sizing and Inspection
Mechanical System and Duct Layout
Plumbing Overview Video
Plumbing System Layout