Except your answer does not include all other places that are required to add a new field to the business transaction. The developer will then end up putting the field in manually and then the business will ask him/her to add the field to the search, BI extractor, middleware and other places and the developer will face much more work and fail to meet the requirement. You just then caused this developer more work because you answered the question, but didn't find out their real problem and offer a real solution. Besides if someone is on a tight schedule and is on EHP1 and above AET is the answer. On the otherhand the question itself has only been asked several times over many years, so probably is a basic question. A strict moderator probably should have locked the thread and moved on 
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I had a SAP CRM 7.0 system with manually added fields with no AET and we had to do extra work when needing to extend the search or BI extractor or middleware for custom fields since we used the manual route for those fields. That being said below CRM 7.0 EHP1, manual is an option for certain cases, but should be limited when EEWB doesn't work.
Take care,
Stephen