E-Business Suite R12: Shared Application Tier File System

In Oracle E-Business Suite Release 12, one of the eye-catching new features was the Instance Home, or $INST_TOP. The result of this Instance Home is that you can now share the APPL_TOP between multiple installations of Oracle E-Business Suite.
I have seen a number of articles and discussions on the Internet discussing the reasons why one would want to share the APPL_TOP between E-Business Suites.

Why share?
Personally, I didn't see any reason why I would want this, other than to see it work and be able to use the feature, when time would come that I would need it.
I cannot remember where anymore, but I saw a reason lately that actually made sense to me:
As a DBA, it could be of use when I would want to replay issues that have been reported to me, being sure that I would be using the exact same APPL_TOP. I could set up a separate E-Business Suite, sharing the APPL_TOP, but having my own database. Now I can play around with my own E-Business Suite, without making changes to the production data, but I am still using the very same code as in the production environment. To make sure I do not break anything, I could even mount the APPL_TOP read only.
Recently I found another reason why it could be interesting to share the APPL_TOP: When your production environment consists of multiple E-Business Suites. I heard about Oracle's recommendations to separate Advanced Supply Chain Planning from your OLTP E-Business Suite environment. You would have to maintain two E-Business Suites, and I can see a couple of reasons why you would want to have identical code versions on each of them. In order to keep the two (or any number of) environments synchronous, you might want to implement the shared APPL_TOP.

Share what? APPL_TOP vs. Application Tier File System
By the way, can "shared APPL_TOP" also be read as "Shared Application Tier File System"? I have taken up the habit of asking whether APPL_TOP means Application Tier File System, or it just reflects the $APPL_TOP directory with all its subdirectories. It happens to be that the two are not the same, even though many of us use the two simultaneously.
In my opinion (but that may be because my experience goes back to the days these technologies were introduced), the APPL_TOP is just the ($)APPL_TOP: the top level directory that contains the product tops (/whatever_directory/apps/apps_st/appl).
The Application Tier File System is more than just the APPL_TOP. It contains the whole set of code that makes the Application Tier, i.e. the APPL_TOP, COMMON_TOP, but also the 10.1.2 and 10.1.3 ORACLE_HOMEs that reside under /whatever_directory/apps/tech_st. To conclude: The Application Tier File System is everything that resides under /whatever_directory/apps. That is a whole lot more.
But hang on? The question now is what Oracle means by stating that the APPL_TOP can be shared among multiple E-Business Suite environments. Do they mean just the APPL_TOP, or the Application Tier File System?
The official answer can be found in Metalink Note 384248.1, Appendix B where you can read that it is possible to share the complete Application Tier File System among multiple unshared E-Business Suite Databases.