Why is it a bad thing? Because she spends about a year away from her medical training, with no reinforcement of any kind mentioned or even hinted at. And that's before her year on walkabout. Taking off for the first few trips with the Doctor, basically up through The Lazarus Experiment? That's a vacation. But when she demanded to become a regular member of the crew rather than remain a guest? That's abandoning your studies to go running off with the Doctor.