The justification for maintaining a license of AutoCAD MEP is this: When a BIM job comes up, you're prepared.
The term "BIM" has many different definitions depending on who you ask. Most people in the contracting business think "BIM" is 3D modeling with clash detection. For a general contractor, BIM means complete project scheduling and management. For the engineering firm, BIM means each discipline in each department collaborating together, sharing necessary engineering information and coordinating a building design efficiently.
For the contractor, all you need (usually) to provide when BIM gets involved is a 3D representation of where your sh*t is gonna be in 3D space. But some General Contractors are requiring Revit to be used (especially since Revit Server has launched) to offer live real-time coordination that they can visualize and control much easier than accumulating exports from everyone and throwing them in Navis for running a daunting interference report with weekly meetings. Also, COBie and QTO is coming into the mix which is putting even more strain on the contractors.
What's good about AutoCAD MEP, is that it's still AutoCAD. So you can decide for yourself to use as much or as little of the "MEP" part as you need. Revit is a different story, however. Be sure you fully understand that Revit is not AutoCAD by any stretch of the imagination and it is extremely hard for Contractors to justify using Revit in most cases. The biggest reason why is because AutoCAD MEP already comes with a ton of content right out of the box, where as Revit doesn't - you are responsible for creating basically everything from scratch. What is good about AutoCAD MEP is that you can always export as an IFC file which is more than acceptable for importing into Revit, so sharing your three dimensional as-builts are easier than you would imagine.
Hope this helps some.