"BIAB can classify instruments in Realtrack, then we don’t need to use these VSTs in BIAB at all!"
It's silly to assume that BIAB can do everything. It has a very decent library, but other Virtual Instruments bring other flavors to composition.

"Take the chord switch function for example. It is such a simple thing. Isn't it just drawing a line in MIDI? Is it very difficult in other DAWs? ? Obviously it is too simple. But in BIAB it becomes begging and thanking? Now that it is done, how many people will use it?"

No, it's not difficult, but you will not be able to audition Real Tracks together with other VST instruments, unless you export WAVs to DAW. With keyswitches you have a luxury of staying longer in BIAB and experiment with different BIAB+VST content, until you are ready to settle and export to DAW. I don't do tutorials, this was just a concept shown to group of forum members. I am sure, if a proper tutorial would be made, more people will use this feature. It is small, but definitely is very useful. By the way, far not all DAWs have a "Chord Track".

"Are these guitar sounds in UJAM not available in BIAB? ? BIAB not only has them, but the sound is better!"
So why even put these word bubbles, if you have no interest in these VSTs or Chord Track. Seems like the choice is very clear for you.

P.S. You do realize that there are many very different virtual instruments from different makers that use keyswitches acoustic, electric, synthesized, etc.? Do your homework and take care.