Media source TVs: predefined output, such as classes, good or bad?

I've never actually used an image TV except for testing an extra I'm writing, but then I seldom work on image-rich sites.

Indeed. The only image TVs I've ever worked with were MIGx multi-image TVs. And those use the getImageList snippet with its own tpl. But the few times I have used regular image-type TVs I either displayed them with a custom output formatter (a snippet with its own tpl) or did my own HTML for the img tags.