Having said that, it is important to stay as current as possible within the limits allowed by your applications....
Some vendor products will only certify certain versions of Oracle so you must stay within those requirements. You will also notice that critical patch updates now being introduced on a quarterly basis from Oracle will only work on the latest versions. This is another good reason to try and keep your database at a current version.
With respect to your question of removing applied patches, you need to read the accompanying readme.txt files associated with each upgrade. They will usually indicate what you need to do to back out a patch or downgrade a database. In some cases (depending on the patch), the database can be downgraded or the patch can be rolled back. In other circumstances, the binaries and databases must be fully recovered to a point in time prior to the patch application.
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