If It Ain't Broke, Don't Fix It
If your organization is applying a waterfall-oriented (or any other) process and has high success rates, productivity, and so forth, don't change. Adopting an iterative or agile method should be motivated by a challenge, not method-du-jour fads.
The average case within organizations is relatively high failure rates and other undesirable project qualities [Standish00]; it is in this context that IID methods are worth considering.
Failure has several meanings. For example, at Symantec (now adopting XP on some projects), the company was attempting a waterfall model. Tight project management resulted in delivering a product on time and in budget, but it was the wrong product [Bowers02]. They adopted XP, which led to a better targeted product; productivity and defect rates also improved.