Hi!
If you use any multi instruments the faders in Logic control the whole VI instead of only the one channel, so all your instruments on all channels will change volume with CC7. This is why I insert a transformer in the environment which makes the sliders effectively send not CC7 but another CC for volume. As you cannot (easily) add a transformer to the "regular" multi instruments, I chose to use external midi tracks, which also allows me to only activate the MIDI channels I really need. Behind the scenes, the Logic multi instrument activates all channels. But if I only want 6 channels, with external MIDI I can only activate those 6 and leave the others switched off. Minimises interference.
Furthermore, with the regular multi instruments, you have the first channel as well as the first output on the instrument channel, but for all further midi channels you need extra tracks and auxes. It works, but the first channel looks different than the others. I want it to be as easy as possible which is why I don't use MIDI channel 1 (which is on the track the VI is instantiated on), but start with 2 and use external midi tracks. Those allow me to cable everything to my liking in the environment and so far I could not find any disadvantages. Because channel 1 is not even activated, there is no need to have the VI slot visible, which is why I hide it. It is out of the way and I only see my instrument tracks. Why would you want the actual plugins to appear on the arrange page? They are VE Pro plugins, which you never need to touch after they are set up. I felt these tracks are visual clutter, so I am glad to get rid of them.
There is one last reason: Those multi instruments you use only came with Logic 8. My method works with earlier versions, too. This wasn't a big decision to use it as I use Logic 9, but I had a lot of people following the tutorial who still use Logic 7.
As for bounce in place: Yes, you cannot do this with external midi tracks, but I have never missed it, actually. I didn't even think of this. But you are right, I will put it in the next update for the tutorial.
Hope that helps :)
EDIT: I forgot one thing: If you use the transformer to route the fader to a different CC, you can easily make the fader control a parameter just by changing the CC. So if you want to use the fader for, say, breath control, remap it to CC2 in the transformer. This only works with external midi tracks, if I am not mistaken.