Fair points, but as you can see from your explanation, it is a bit more involved and not quite as simple and straighforward as one would like it to be.
Nowadays, I only do a few mail merges a year now, so I am afraid that if I took the time to learn it, by the time I would need it, I probably will have forgotten most of it. Like so many other things, if you don't use it regularly, you lose it!
Word interrogates the first 8-15 data rows to 'guess' what kind of data each field contains and, if the data are mixed, or missing from some rows, Word sometimes gets it wrong. Formatting all the data as text makes little difference to how Word interprets it, since the formatting is ignored.
I can't say that I have experienced this behavior myself. Every time I have used the TEXT function to format it, it comes out that way in Word without doing any sort of re-formatting. As matter as fact, Word "guessing" the format is usually more problematic, because as you say, it often guesses wrong.
One of the advantages to my method is that I can view all the data in one placel, and know that is how it is going to show up in Word (at least that has been my experience). So I can easily spot and data issues simply by looking at one data file. If I do switches, and somewhere down the list, Word guesses wrong, and puts it in a bad format, I would not be able to see that easily. I would need to look at each and every merged field on each and every page. Some of our merged documents are over 1000 pages long, so that really isn't too feasible.
Microsoft Access has the same format "guessing" issue when trying to import Excel files. And their error reporting is awful. All that it does is tell you that there is some sort of error, but gives you no indication of which field or record it is, and then it doesn't import anything. You can spend hours trying to figure out how Access is reading it, and trying to determine which field/record are causing the problem. Inevitably, it is usually quicker to export the data to a Text file, and import that into Access where you get to specify what the format of each field is.
That is my major complaint with the MicroSoft Office products, especially with Word. They try to make these things so user friendly, that they try to do most of the thinking for you. And inevitably, they guess wrong enough times to drive you crazy. As a programmer, my mind-set is I want the applications to do what I tell them, not try to guess what I might want to do.