OK thanks folks, the concept is starting to get clearer to me.
Chocky, for most of your questions I was considering the extra things to be modules, so perhaps this explains my confusion.
"The questions of mine which you've quoted are largely rhethorical, so I won't answer them directly, but I think you can answer many of your own questions youself."
Heh, most of my questions were also rhetorical, the point being that having a special manager wouldn't magically solve these problems. If I've not got a new enough shared C library (which is usually my problem) then on my net-less riscPC there is nothing it can do except ask for me to get it (or as you just mentioned, include it in the package, which is not done in many cases for legal reasons).
"But referring to one question you ask "Because it if does, it should tell you". Which "it" would this be? Again, we have to resort to an adhoc system. And a program can't always know that it's missing part of itself. It might just crash. Better to have one system that avoids this."
You see I was thinking of the case where you run the program and it says "cannot find component X" (or update your C library or summut). If the additional file is part of the package as well then I can see how the package manager could check to see if it was present and install as necessary. And if the missing components are apps rather than modules then it makes even more sense. Although on the other hand this might mean that the zip-files will get much more bloated. Since I have to transfer everything from work, (and this used to be via 1.44M disk) you can see the problem.
"I'm afraid I can't help feeling you aren't really following the discussion, but are just picking up points slightly at random to comment on."
Perhaps he is, and just didn't understand (like me). If you are not deeply aware of the concepts of Unix or PC systems then describing it with reference to them doesn't help, so I(/we) have been struggling to understand on the basis of how RiscOS works.