Sorry if this has already been addressed but I couldn't find.
I checked my DCBackup file and had literally thousands of CC I've downloaded since TS3 came out and 18 dbc extension files in DCCache!!!! It's been quite a chore to clean house.
In case you have not come across it, here is the Sims Wiki explanation for the DCBackup. I delete the DCBackup files with the exception of the comerged.package file described below.
DCBACK UP:
Contains all package files that get unpacked when you install a sims3pack. This folder is bound to have many files if you install and test out a lot of content, for the files in here will not get deleted when you uninstall or delete something in your Launcher. The names in here are just numbers so you won't know which is which. To find out, you can open them in s3pe and find the picture (Type: THUM or ICON).
If you export a Sim / Household / House from your game, the game only packs the stuff used that is actually in this folder. Meaning if you used a window in your house but deleted the corresponding Package file in this folder, the window will not get packed with the house. Some Downloads don't need to be used to get packed. They just do, but only if you have them in this folder.
Since the introduction of premium content to the store, this folder contains a file called ccmerge.package. This file contains information about the buffs and skills given by the premium content. Don't delete this file or it will cause the premium content to not work correctly.