Hi,
I always compress my packages (full and updates).
Indeed, the parent package is not recompressed after an update package, so from time to time, i recompress the full package (there is a procedure to do so, using the package build history), otherwise when you install a fat client, it uses the old compressed files.
Furthermore i was really happy to use compressed full package a few weeks ago :
I killed an update package during its build and it seems it corrupted my parent full package : i couldn't build any update package (i get errors in a tam file update in the parent package). Hopefully i had recompressed my full package a few days ago. So what i did was to get the spec files in the compressed archive to replace the non compressed files. It saved my life !
Even if your compressed files are not up-to-date, you can use them to replace a corrupted file in the parent package. After replacing the wrong files, you just need to relaunch the build of the client package (using the build history, selecting the client package and click on "resubmit"). As you didn't change the status of the package built, it won't completely rebuild the package, it will just update the parent package. (To check that, just try and take a look at the log files of the update package).
Hope this help.