hello brucerez,
Yes, for *.cqi (*.pqi) all previous content must also be selected. It's like you would say "that's what I wish to have installed at the end". However when installing this *.cqi, only newly added content will be read (added to existing pack in keyboard). As said, that only saves install time -*.cqi file itself will grow each time you add something (the same way is it would with normal *.ppi/*.cpi file). Again, previously installed content (on keyboard) must match the content in *.cqi file + added content.
Practical example of when quick install option can be useful:
You made a pack which has 3 styles and 1 voice (which is used by one of the styles). After you installed this pack, you realized, you forgot to select one more voice that's used by another style. What you do (in YEM) is, additionally to already selected content (3 styles & 1 voice) you also select that one missed voice. Now you create quick install file (which now contains 3 styles and 2 voices). During installation, only that newly added voice will be read into keyboards memory -which is faster than reading whole pack. And if for some reason previous content (on keyboard) doesn't match the content in quick install file, then the whole pack content will be installed (same as in the case of normal pack) -because "that's what I wish to have at the end".
So... is it worth to bother with quick install packs? Depends. If some huge pack is already installed (which you wish to keep) and you only wish to add a voice or two, then the answer is yes. But.. "who would keep a track of what's already installed and what not..." -well, there's no need for that: if previous content doesn't match, then you end up the same way as if you would by installing normal pack.
Speaking for me, I don't use quick packs... main reason being, SX700 memory is kinda smallish and so there isn't that much benefit.. and I'm never in hurry. But in case of SX900, which has a lot more memory, this can be quite a time saver.
Bogdan