I don't think the idea is to force developers to do anything, especially when it comes to the front-end appearance..
I am 100% an administration that has unity in how it looks and feels though.. without question! it's always tricky jumping from extension to extension on the back-end when you install any that do their own thing and don't stick with the UI conventions you're familiar with.. extreme colors, strange layouts, loud graphics ...The administration should flow and look unified for sure.. that is where I see the power of the new JUI framework.
Front-end design is something that obviously varies from site to site.. so I don't think the idea is to force you into a canned look on your site front-end but rather provide a convention that makes it easy to set up the style sheet and know what to expect when you install a new component..
As it stands now, when we design a Joomla template where we work ( which is always a custom job, we don't resell templates ) .. we always build in time for styling extensions we didn't write.. We know we're going to have to tweak things that are third party mostly because they often don't stick to a convention... it would be GREAT if people used a common set of tools so that it would be easy to snap in a css file and know that things are going to play nice visually without having to hunt them down and add in a bunch of otherwise un-necessary css because they decided to use a class name that makes no obvious sense =)
Unfortunately though - having the toolset won't force it to be used .. one can hope

I just hope this doesn't scare front-end designers into thinking this is going to force them to have a UI that looks like every other Joomla site .. I'm confident that's not the idea.