ccing @kevinator since this might apply to his team, too.
In my team, we like to estimate stories before they make it into a sprint. As the Product Owner, then I can have a somewhat realistic sense for how many tasks can fit in a sprint and keep on pace with the workload of the engineering team. Without being able to add points to a task before it gets into a sprint, this isn't possible.
I'm not sure what the best solution is, since we probably don't want this property to be showing by default on all the tasks in phabricator, but wanted to open it for discussion.