When I hear this argument I believe the concern is not the syntax of the binding with respect to the HTML spec, but rather the declaration of bindings in HTML at all. So the issue is not "ng-click" vs. "data-ng-click", but that "ng-click" in any form is no different than the pre-JS MVC horrors of onclick="mutateGlobalStateAndPerformIO()"; which is precisely the barbaric approach we thought we were vanquishing. I think that some people prefer to see bindings sprinkled on the DOM via code, as part of the bootstrapping process. I haven't used Backbone for a year but I'm pretty sure it is a strong proponent of "let nothing live in the DOM" school of thought. I don't subscribe to that, by the way, but that's the argument.
In Angular HTML is the view so that's where ng-click and friends go, while other MVC frameworks have view js files. I know people like to be 'unobtrusive' but many JS MVC apps would be useless without JS so there's nothing to fall back to (other than a plz turn on JS message).
I dislike data binding with a vengeance. From Windows Forms, to Web Forms, to WPF, to Swing, to QT QML, to AngularJS. I don't think this pattern will ever work for me.
What happens when you want to add a second, unrelated, thing? Just stick it on the end in the onclick? Create a new function called doTwoThings()? This is one of the reasons I prefer using an observer pattern over MVC.
That doesn't really happen though - also, Ng-click is a specific directive you can choose not to use.
A more general argument is that any behaviour specified in markup is bad. I'd counter that by saying that it's already there in HTML. Input boxes for example have behaviour intrinsic to their being. So angular let's you create a (for example) richtext tag. You attach behaviour to that tag, say on click. In the HTML it's as clean as <richtext>. Within the directive you can make it behave as you please.
I think that people see the shortcut directives used in angular (eg Ng-click) and the purists shout "unclean"! The reality is that You can do things a slightly longer way and end up with something thy looks a lot like the observer pattern.
Nothing, but some feel rather strongly averse to it, and plainly view this as Angular's self-evident fatal flaw. As I said I do not subscribe to this line of thinking. My allusion to the horror and barbarism of the pre-JS MVC was merely a jape.
