QuickTime incompatibility with earlier versions

Question

I run a tech support department for a software distributor. Some of the titles we sell have versions of QuickTime going back to 2.1. We spend a huge amount of time supporting customers with QuickTime issues because the program is not backwards-compatible. The earlier versions, say 2 - 4, offered the option on installing to "look for other versions of QuickTime," which you could "Skip" and then you could install different versions side-by-side on the same machine which worked fine - each program would look for its particular version.
I believe version 6.5.x was the first version of QT that, when installed and updated, seemed to work with the programs using earlier versions. Then series 7 arrived, which insisted that you uninstall any other versions, and it was not backwards-compatible...software developed with prior versions would not display the multimedia content.
One of the publishers we distribute programs for has a 2007 and 2008 edition of an encyclopedia that uses the 7.0.4.80 QuickTime. This version, once installed, defaults to automatically upgrade to the latest version of QT, but when users update the program to 7.2, they receive an error message that QT won't play MP3 and MP4 files. The only fix that we've discovered is to manually replace one of the Jar files. Apparently, a file in the 7.0 verion of QT is looking for that specific version of QT.
We have hundreds of software titles that won't run on a system with the new QT. Some of our publishers that use QT are as concerned with the compatibility issues as we are, and are considering alternative programming for their multimedia content. We've spent countless hours trying to offer support to users who are frustrated - either that their current software is no longer working, or that new purchases are experiencing QuickTime problems.
Occasionally we are forced to recommend uninstalling the latest version of QT and reinstalling a prior version, at the same time disabling the option to update the program. We are looking at the possibility of discontinuing distribution of programs that use QuickTime.
If anyone has any information that would shed any light on this situation, we're open to suggestions. We are in a unique position to assist in resolving this problem since we have several programs using different versions of QuickTime, and we're also to communicate any antidotes that may exist to the other publishers we work with, so any feedback would be greatly appreciated!
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Answer 1

What Kirk says is generally true so far as QuickTime support for earlier versions of QuickTime media.
That said, I wonder if there is another set of problems here. Could it be that the media is supported, but the system calls are not? QuickTime for Java, for instance, has come, gone, and come back again, and in any case does not support the complete QuickTime API.
Another problem I have seen is that the default settings for the QuickTime installer seem to change periodically, and it affects the types of media that QuickTime will handle by default. Part of that is because in an early version, QuickTime did the same thing that RealPlayer does now, taking over ALL media types whether it could play them or not. Now the installer is much better behaved, but any application that just opens a file and assumes QuickTime will handle it, as opposed to opening QuickTime and using it to open the file, will break. Likewise, in the name of "security", Apple has recently removed some of the interactive features from QuickTime (why was it Apple's responsibility to fix a lazy programmer error at MySpace, anyway?)
The fact remains that QuickTime is the best multimedia platform out there, but there have been changes, and my guess is that the most critical changes are not in the support for older media, but rather in the API.
--Dave Althoff, Jr.