Normally I tell designers: Don't be derivative. Don't just copy the dominant UI because it happens to be the dominant one. Think the design through from first principles of human cognition, visual design principles, usability heuristics.
OmniGraffle. Who are your users? I'm guessing professional knowledge workers. The only people I've known to use it are professional designers, maybe a product manager or two. These may be people who know HTML and its structured styling examples. These are certainly people who have some basic word processing / style examples. In this case, I am in fact one of the target users.
There is nothing about this style manager interface that relates even remotely to anything I've ever seen before in any desktop publishing or graphics application:
Click on "Other", like you'd think you could if you wanted to edit, manage, or change a style. You get this totally bizarre dialog box that pops up in the middle of the screen... and god knows what next.