Let me try one more time, then I surrender...
First, you can't "mix" DTS or DTS-HD MA. Those are encoding schemes or methods. They take 5.1 or 7.1 (or 2.0) channel already-mixed music and encode them so that your receiver can decode them later. Mixing involves taking multitrack recordings and combining the individual tracks (or more correctly "the stems") into a 2.0, 5.1, 7.1 or whatever channel mix. In the old days, mixing boards were used for mixing. Now a combination of DAWs, such as ProTools and mixing consoles, such as this: http://www.avid.com/US/products/Avid-System-5
The reason the LFE exists is headroom. When you are mixing there are often times you want to add bass emphasis, whether it be a bass drum or low note on a bass guitar or a synth. In order to add that emphasis, you would then have to lower the level of the other 5 channels. Doing this reduces the dynamic range of the mains. So, instead the emphasis can be added as needed in the LFE without affecting the other channels.
Also the use of that LFE channel in SACD as a main height channel (see http://www.sa-cd.net/showthread/160/2429
for old discussion) is another way the LFE channel can be used discretely.
Yes, if you have a 5.0 channel source and your system is properly tuned, all that will come out of the subwoofer is a low pass of what would have gone into the mains. The mains then get a high pass version of the signal. This assumes that the mains are set at "small". Your ML system also does this since the subwoofer handles frequencies (ML subwoofer-only frequencies in the car are usually below 50Hz not 80Hz) that the other woofers cannot handle.
So, I'm not arguing with you that the method you describe doesn't happen. What I am saying is that it doesn't always happen. It is up to the recording engineering, the remix engineer (in a movie), the mastering engineer and the authoring software on how the LFE is presented. It isn't only one way and there are many examples of the LFE being used as "discretely" as the center or surround channels.
Finally, my point on the "Surround" button in the ML system is that it should be disabled before playing a discrete 5.1-channel source. Otherwise you get pseudo-surround added on top of surround and you lose much of the localization that the ML engineers worked to put into the system.
BTW, I really enjoy my ML system and use it mostly with DVD-Audio discs (at least for 5.1-channel sources). The only system I've heard in a car that was better was the premium system in a much larger Mercedes Benz vehicle.