Based on my experience, dynamic languages without optional typing make large scale enterprise development unbearable.
Sure, one should be writing unit tests, but in the enterprise context those tests rarely do exist, or if they do, either don't test what they should or are so complex that invalidate any re-factoring taking place.
I'm curious how many runtime errors persist due to the optionality of the type system. Perhaps code standards requiring all "library" code to be typed would be a good balance.
Most enterprise customers I have worked with, favour dynamic languages just for scripting tasks, while using static typed ones for the large scale projects.
By large scale, I mean, projects with at least three development sites, at least 30 developers, all the different set of skills. With lots of attrition.
The only time I saw it working, was in a project done with TCL, which lacks optional types, but everyone on team was a top developer, the team was small, and located on the same open space. So startup world, not enterprise.