So, my feeling is this: it's hard to dispute that joins are core; almost anyone using joins is going to run into problems with skew at some point, and ask how to solve that; having a good answer for that seems pretty fundamental.

In the very earliest days of Scalding, one piece of FUD that was often directed at is was "but Pig has SKEW JOIN, we can't take anything seriously if it doesn't have SKEW JOIN". I think having a definitive reply to that counts as 'core'.