Art that makes things easy can be easily ignored. It’s only when things are unclear that art can demand attention.
Granted, this only works when the reader (for the sake of simplicity let’s assume we’re talking about literature) either really loves or really hates what they’re reading. If they love it, then naturally they’ll want to defend it, and if there’s some nagging ideological point that’s not quite clear, they’ll want to come up with an interpretation that lands the novel on their side so they can enjoy it guilt-free. And if they hate it, then maybe they’ll want to back that hate up with an argument that it’s offensive not just to the aesthetic sense but to the moral. In either case, we’ve got them thinking critically, and that’s better for everyone than the readers just putting the piece down and forgetting about it. That’s my case for ambiguity, and that’s the sense in which I think art should be provocative – transgression must be subtle and to some degree up to the reader. Being blatantly and unilaterally offensive accomplishes nothing. And of course, it has to be fun to read and readable too, or else no one would read it through to the end who isn’t already beyond help.