Actually, I know a number of users who do just that--they stick with AutoCAD and draw lines, circles, and arcs. I know some other designers who have ADT and/or ABS (MEP), but use it as plain vanilla AutoCAD. Why? Because when you get a budget for a project, it doesn't include time for training, building content, etc. In the case of designers who have Revit, many of them just don't use it. It's all economics.
So, why do these designers buy Revit? I bought it because it was a good deal at the time. The vendor told me that it wasn't quite there yet, but I went ahead and bought it because ABS came with it, and I figured that sooner or later it would be usable.
Other designers end up with Revit on their machines for other reasons. Here's one scenario. Instead of pitching to its customers--A/Es and other designers--Autodesk pitches to its cusomers' customers. They make a pitch to the Corps of Engineers, GSA, Department of the Navy, Department of the Army, Department of the Air Force, and a host of Fortune 500 corporations. They put on a slick demo, admittedly canned and well-rehearsed, about all the benefits the owner will realize if their consultants use Revit. The Corps, etc. foams at its collective mouth, and issues a decree that all design documents will be prepared with Revit. Any firm that wants to do business with these agencies is forced to buy Revit. The principals that buy Revit don't actually have to use it, or even know how to use it--they just pass the decree on to their designers. Case in point--in the 70's and 80's, you had to have Intergraph if you wanted to do work for Kentucky Fried Chicken, at $60K per seat.
As far as plumbing goes, you are correct--plumbing seems to be the weakest part of any MEP package, and Revit MEP is no exception. However, the plumbing tutorial destroys any credibility that might exist. Although the tutorial does illustrate a few techniques that one would probably not discover by accident, the system as modeled would not pass any plan reviewer's scrutiny. Most reviewers would end up on the floor laughing if someone submitted drawings of this system. There are no individual vents. Some fixtures are wet-vented. The sinks don't have traps. Should new users be able to rise above such trivial points? Probably. But, the fact is that most people don't respond to contrived examples as well as to realistic ones.
As far as being negative in forums, how else can you make a point? Autodesk isn't going to fix problems if they don't know they exist. And, believe you me, there are problems!