A layer of dust or dirt on top of your insualtion would be quite normal in a vented attic. If the dirt was inpregnated in the actual insulation, so that it was acting like a filter, then that would indicated air movement from one side of the insulation to the other. This would be seen most often around holes in the ceiling from light fixtures, plumbing/electrical or any areas open to air leakage from the interior to the attic. I think what you are seeing laying on top of your insulation is just plain old dust from outdoors entering through the venting. not a problem.
I realize this is an old question, but it's hot once again and the question may come up for someone else.. so I thought I'd venture an answer. I don't work as a heating/cooling professional, I'm just a homeowner.
Your plumber may be right, and from my experience, it is uncommon that he'd know that (unless, like you said, he was trying to get the home warranty company out of a bill and happened to stumble on the cause).
When your attic gets hot, and there is bad or no circulation, according to Charles' law, the pressure in your attic increases. If it gets hot enough, it can increase high enough that it can suffocate the pilot flame. So if you have no circulation in a hot attic, you can definitely snuff out the pilot flame. Replacing the water heater may not fix the problem. But probably neither would circulating the high pressure air in place with a box fan.
Check your venting situation, especially the passive elements. For example, if you have soffits, make sure they have not become clogged with blown insulation. I would take a blower of some sort (a shopvac that can be reversed for example) and blow them from the outside. Also, look from inside the attic and check what the situation is. If you've had recently blown insulation, it could definitely have caused a problem.
So my suggestion - clean your soffits and evaluate your passive ventilation. Resist the impulse to add active ventilation unless you know what you are doing (they can cause issues). If you can't figure out the problem, it may need the experience of an experienced roofing contractor who understands how to calculate your roofing needs and install additional ventilation. It could be that your two turbines are sufficient, but you are not getting enough air in through soffits and other inlets, for them to pull out. You can google for various formulas to consider how many feet of soffits or ridge vents/passive ventilation you need. If you have no soffits and only a non-functional active fan to push air in, that's probably your problem. I'd add soffits.
By the way, it is possible that as your water heater has aged, it is more sensitive to the increased pressure. This I don't know, I am just theorizing, and someone with practical experience should chime in. Replacing parts or replacing the heater may help or fix the problem, if you are just on the edge of the issue. If you've already fixed the issue, it would be interesting to learn what solution you finally arrived to.
Best Answer
Wow, landlords can really be dummies sometime. The simplest answer is that the turbine company paid an engineer to design/improve their turbine to be most effective relative to the cost of production. If stopping the blades were more effective then there would be no bearings for them to spin on and the turbines would cost less to build. The landlords idea could be improved by taking the turbine off the base and leaving the attic open to the sky. This, however would allow rain into the attic just as what would happen with a driving rain with stopped blades. Empirevent.com has a nice website with photos. You can easily see into the interior when the vent is stopped and that is the path for rain to enter.