Marx was never as influential in Economics as he was in Politics. His political philosophy is interesting and challenging while his Economics always seems chosen to suit his politics. It wasn't bad compared to the Economics of his contemporaries, but it had a short shelf-life. He lost the battle of ideas pretty rapidly, and fairly in my opinion.

True, one will be a better Economist if he understands the evolution of Economic thought in the 19th century, but there is no reason to single out Marx for particular study by the average student.