[sirinsidiator]

A) AddOns with huge amounts of data already do that. HarvestMap, Master Merchant, Arkadius Trade Tools. They all use more than one file to separate config from data and also offer ways to purge that data. For most other addons it just doesn't make sense as it is complicated to set up for no gain.
B) Purging data is not something that you can do from the outside without messing up the addon, or making it useless. As such I cannot recommend it. Even if you did make some tool that knows each addon well enough to delete parts of its data, it will most likely lead to issues in the future whenever the author decides to change the data format, so again not something that can be recommended.
Saved Variables of addons which are no longer installed don't have any effect on game performance. They are simply ignored as if they didn't exist.
C) That's the minimum functionality I'd expect of a tool that deletes data it doesn't own.

Overall I don't think it makes much sense, since the data is very addon specific. You cannot make any decisions about what to delete and what to keep from the outside.