I wonder how many users of DSLRs use MLU? I wonder too how many have actually heard of it?
It seems to be that a lot of sales in recent years have been made to those who want better image quality, and want this applied to their family and holiday photos. If this is the case, why would they need MLU? Further, any books about 'Digital Photography' they are likely to buy may not mention it. I suspect a lot of people just want better images but don't expand their photographic depth. This is not a criticism, it's just an observation with a lot of assumptions.
From this angle, adding easily accessible MLU may seem a waste of time. Ironically, LIve View may be providing it in any case...
The business mind in me gets the manufacturer position. The photographer side is irked, because it would be great if manufacturers provided such things, and manuals that described the opportunities provided by cameras. Instead, manuals simply list instructions for how to do things which are obscure, and to which no photographic advantage is attached. If manuals encouraged use, the manufacturers might sell more supporting gear such as lenses, flash etc. Oh well.
Anyway, it still irks me that my 5D, often mentioned as a good landscape camera, does not have a dedicated MLU button.