An IDE does more than edit, compile, link and debug.
it does neither of the above

There are a number of software engineering functions that an IDE handles quite well. For example, source code management, keeping track of changes and a documented history of editing. Another is project management for the lifecycle of a product, where the IDE can show tasks and assigned programmers (very handy if you have to track down what someone else is doing).
I know no '51 IDE that does any of tha above

Granted an IDE isn't necessary in a one man shop, but as any organization grows there has to be a standardized way of doing things so continuity is preserved through employee turnover. An IDE serves well in this capacity.
that ia false as well, an IDE does not enforce standardization that occur not through the IDE but but 'rules'

Erik