to extend a schema or not? cons and pros

To extend a schema or - that is the question... Yes, what are the positive and negative aspects of this process:

• positive - You will have just custom - prepared for Your environment classes and properties
• positive - You will avoid some unexpected behaviors like - You hid some attribute - because it is sensitive an after some patch or fix it is now - again - visible for everyone
• negative - it is irreversible - you couldn't fully remove it from You schema
• negative - it is not standard property or class - somebody will ask You - how it is possible, that You have some custom attributes, but You don't use standard attributes?
• negative - it is dangerous - just kidding - it is just quite normal operation so it is neutral for every environment of course You should sit on Your hands before the change, test and rethink names, but it is quite normal operation - of course as long as You don't have a plan to add attribute called "DisplayName"...