Marketing: We want new demographic 'X' to give us their money and/or
do this thing. What should we throw at the walls to convince them?
Look, shiny! Funny! Cute! Oww! Money! Sex! ... Ok, which one
worked? Let's do that one again. We want new demographic 'Y' to give
us their money and...
UX: These people from groups A, D, Q, X, and Y have already shown a
willingness to give us their money and/or do the thing. Let's
entice them to stick around by giving them something they want or need
in a pleasant/entertaining repeatable way. Let's identify and fix the things they don't like to do.
Both: We want the people who give us their money and do the thing here to keep
paying us and not go do the thing elsewhere. Their money
and continued doing of the thing is important to our employment, and
it is based on user satisfaction or necessity. We should figure out
how to improve user satisfaction, or increase their need to do the
thing, while still getting the money. Let's share information in these areas!
Good UX can attract & keep users, but its purpose isn't to find them in the first place, get them in the door, or determine how much they're willing to pay compared to competitors. Marketing's job is to be proactive, experimental, and focused on expansion/profit/customer base.
UX can be proactive in the use of best practices and good design, but is primarily reactive or pre-emptive in terms of customer satisfaction and retention. The chief concern is whether customers are willing to continue showing up, and fixing the reasons they might not.
Another important distinction is that "the user" and "the customer" are not necessarily the same thing. Facebook, for example, has gone off the deep end in pursuit of profitable marketing strategies, selling to non-user third party customers, and making themselves a ubiquitous necessity at the expense of good user experience. "Users" are fed up & ready to leave in droves, but customers & share holders are pretty happy.
I agree there's clearly a lot of overlap, but I think cooperation depends heavily on the nature of the organization. Could be their immediate goals are at odds w/one another.