The problem I have is that my UIs generally end up with complex requirements. At first, it's just a standard CMS-like site with relatively static content. Then they want discussions. Then they want live discussions. Then they want the ability to highlight sections of the article and hang running side-conversations off of those... etc... Without using some kind of front-end framework, I end up organically evolving my own to tackle the increasing complexity of the UI. Now, I've got UI logic spread across two layers: my back-end (Rails or whatever) and in my front end (my hodgepodge JS), both of which need to dynamically render / update markup-- at which point I wish I'd just gone with an SPA framework in the first place.