How many of us know a product with a terrible UX but became successful in their market? That is when the product functionality overcame the UX. It's the dancing bear, as Cooper (2004, p.26) calls it as when a great idea triumphs over poor design.  The notion of dancing bear overlooks the fact that the bear is actually a terrible dancer. A successful poor UX product is not necessarily a marketing victory. Sometimes the product just offers functionality that is so important (value) to the user that UX fades into the background.

UX and marketing should not mix, except for when you are briefing to the marketing people the benefits your UX has to the users. In the end, it will be the marketing department that will decide if the UX information is valid or not for them. We can say that all depends on the company's strategies and how they want to present that product to the users. Unfortunately, not every company has UX in the centre of their goals. 

And about ROI, your answer is already here in a previous question on this site: What is the ROI of UI or UX design?

Is UX better marketing than marketing?
Why do some people have the custom to wind the Tefillin-Shel-Yad towards themselves, while others wind it away from themselves? What is the reason behind these two customs, and which sects of Judaism generally follow which method?
