I know this is possible too with track instruments, but I think the big picture is much more clear with the Rack instrument.
IMO, I think it really depends on your workflow and use of multi-timberal.
Initially everyone used Rack Instruments because there was no option. So you develop a mind-set based on the past.
Eventually, I used Track Instruments for single midi VSTI's and reserved Rack Instruments for Kontakt, Halion, or any multi-timberal VSTI's. However ultimately after a few years I became frustrated because there were always 2 continual methods being used, and I felt my workflow speed was a bit hampered because of 2 different methods, my thought process, automation, and how they show up in the mix window.
A couple years ago, composers started using hundreds of single instances, claiming there isn't much of a processing difference, so (with the exception of Toontrack where I had invested a good amount of time configuring many templates that only work under Rack Instruments using the Cubase feature Save Selected and Load Selected. Save Selected/Load Selected only work with Rack Instruments) for simplicity reasons I decided to go with Track Instruments.
I'm happy with the Track Instrument method, however Guys issue of enable/disable and Track Archive Export/Import and resulting messed up midi assignments, clearly show a difficult bug which is a drawback. I think the difference between myself and those with hundreds of Track Instruments is that while I disable/enable tracks, I'm not using Track Archive Export/Import combined with Enable/Disable. I have been using this method for well over a year, however disable/enable is not an every-day function since I only use EWQL Gold and limited Kontakt orchestral tools. I'm not a orchestral composer, just mostly pop and integrating occasional orchestral.
To sum it up, I guess it's really how your mind works, what you are used to, and how you wish to address multi-timberal instruments.
Hope that helps.