I have a couple observations, and this includes gasoline-fueled cars as well. If a MAF is totally not necessary, then why do so many (the vast majority) of automakers use them? They are somewhat fragile (especially the hot-film type, like most of our Volkswagens use), kind of expensive, and in pretty much every case where they are used there is also the other sensors that can be used to "calculate" the air flow using math instead of actually measuring it.
Now, the OP says none of the other diesel engine makers use them, but that is not true. Every single one of the road-going diesel engines under 10k GVWR use a MAF sensor. Volkswagen, Ford, BMW, Mercedes-Benz, GM, all of them. And with the exception of Ford using Siemens, everyone uses Bosch EDC. Could this be a conspiracy for Bosch to sell more MAFs?
Keep in mind, Caterpillar does not [yet] need to meet the emissions requirements that Volkswagen does, since they are not building anything that small that is roadgoing. And they probably don't have any engines that are asked to go from 900 RPMs to 4000+ RPMs within a second while still meeting those same emissions requirements.
So to that end, it would seem a MAF is a very integral part of an EPA-compliant emissions friendly engine management system. Also, all of the new diesels I am talking about have throttles. All of them. On a Sprinter, if the MAF wigs out, the ECU cannot operate the throttle and EGR valves, and it reverts to dumbassed mode, running wide-open throttle, closed EGR, and reduced power.
So that gets us back to the original question of "why do tuners use the MAF?" Well, if it is already present, and you are wanting to keep the engine at least close to emissions compliance, then why
not? I don't see how you could accurately operate these systems without one. But if you don't care about that, OR are wanting to build an EDC system from scratch like in the other thread, and perhaps using said home-built EDC system in a swap or some other more unregulated diesel engine, then the simplicity of a MAFless setup has its merits. Especially if you can do it without any adverse driveability or ill effects like smoking, lagging, etc. Simple is always good. If I put a TDI engine in my Vanagon, you can bet it won't have any extra bits it does not absolutely NEED to make the engine operate in a reasonable manner. Heck, even a 75hp SDI engine would be a substantial upgrade
.
So I think if we could get the answer to why some of the OEMs don't use them and some do, we'd have the answer to why the tuners do or do not. The 2.5L 5cyl VAG engines in the current Golf/Jetta/Beetle/Passat don't have a MAF, they seem to run fine. But the VR6, the 2.0L non-turbo engine, all the turbo engines, as well as all the TDIs, do.
Also, as far as MAFs being used for EGR checking: that may be true in some cases, however there are other methods used to verify EGR flow. Temperature sensors post-EGR valve, delta pressure feedback (DPFE) sensors, and even a MAP (on non turbos) can be used. Heck, GM used to use the oxygen sensor signal to check EGR operation. Plus, many engines that have a MAF,
don't even have EGR valves.