It's not just designers who appreciate using declarative markup over programmatic syntax! Of all the people who might understand this, I'd have thought you would be the first in line (as a fan of your work on core.logic and inspiring me to learn prolog)!
I'm not trying to be too non-constructive critical here, I fully admit I haven't tried Om/React, just looked at your blog post about it. So what am I missing?
However, if you use the clojurescript version [or in standard React, the javascript- or coffeescript-looking version], then you suddenly have a bunch of potential extra features which in some cases are invaluable. (Or you can deliberately ignore those if you prefer.)
When you say “declarative” I think you might really just mean “familiar for html experts”. But most html expert should be able to learn the new syntax quite easily, since the relevant parts are less verbose than html, quite regular and straightforward, and semantically equivalent.
If you feel like putting your templates in separate files from your logic, or writing them using an SGML-like syntax, that’s pretty trivial to do in any of these environments.