Why should Mahayana practitioners strive for anything at all?
Well in Mahayana, just like in Theravada, we strive for the end of striving. As @ChrisW said, Heart Sutra is written from the perspective of completion, the end of striving. This is called "taking the result as the path". Meaning, instead of discovering the result step by step, you try the result  on, like a hat, until you internalize it through getting used to it. 

Another point of Heart Sutra is to serve as an antidote for attachment and reification of the raft of Dharma. Mahayana's issue with Theravada is that they sometimes get so obsessed with Dharma concepts to the point that Dharma becomes a problem, an obstacle in itself. So by reminding of its inherent emptiness we are hoping to help the student come back to the natural state, so they can experience the groundlessness and positionlessness in which there is no more need for striving.

Now, to just abandon all effort without having mastered the peaceful mind would be fooling oneself. Which is why in real life most Mahayana practitioners have to work on essentially the same practices as Theravada, whether before or after realization of emptiness.

Wouldn't it be nice if one could attain dispassion and unbinding upon hearing that everything is contrived and absolutely pointless? But habitual patterns and karmic inertia are difficult to overcome... 

Although certainly in the absolute sense the universe does not care if you're enlightened or still suffer. It remains as lacking any solidity or certainty to rely on as it always were.