While writing an IDE in SQL is a great example of Relational Development, the reason our shop builds SQL Server IDE's is because we use SQL Server every day, all day long; with our premiere SchemaSpider IDE:
You can access any schema element instantly on screen at once.
You can drill into, edit and rebuild any object with one click.
You can filter the entire database view on a field name or code fragment.
You can edit a stored procedure alongside a copy of itself or any other procedure with one click.
You can drag blocks of SQL back and forth from anywhere or execute them ad-hoc.
You have several immediate code windows you can navigate with a single click.
You can easily morph the IDE in terms of screen real estate; forget docking 30 tiny little controls into the environment, you can dock the environment into the target control.
You don't have to watch the hourglass.
And if you're running the IDE on hardware that's really old (like a 300 MHZ NT4 or Windows 2000 box from 1998 running SQL Server 2000) you still don't have to watch the hourglass.
Some illustrated screenshots of the SQL Server IDE follow with more information; the last shows a Relational Web Application being launched (in addition to all kinds of SQL Server development, the IDE of course lets you build Relational Web Applications).