The general consensus seemed to be that they would be helpful, but only if they allowed the writer to get at the bare Inform or TADS code underneath and not use only high-level editing.
Also, it was realized that such tools currently don't exist because the people with the skills to write them generally don't need them. There is an Inform IDE (Integrated Development Environment) or three in the works, and TADS comes with Visual Workbench, but IDEs are not what is usually meant by "visual".
However, if you want to write a visual IF editing tool, great. Show us the results. Just don't wander in and ask somebody else to write one for you.
The Quest system [System:Quest] includes QDK, a visual development tool. However, Quest isn't quite as advanced as Inform or TADS, requiring you to code just about everything from scratch. SUDS [System:SUDS] is quite visual; however, the games it creates do not have a parser and are more like a point-and-click game.