Edit: My inference that Quirrell is overwhelmingly vulnerable to Dementors seems to be incorrect, explanation here, although he is more vulnerable than usual. The importance of keeping Patronus 2.0 up derives from it being a blind spot for Dementors, allowing prisoners to escape. Enough time without Patronus 2.0 leads to impossibility of prison break.

Ch. 54. Since Quirrell was that unusually vulnerable to Dementors, he should've made the point of how important it is to keep the Patronus 2.0 up at all times and immediately restore it in case of failure, making that idea more available to Harry's mind. Not making that point explicitly was stupid.

(Quirrell should know that, since Auror's Patronus utterly failed to shield him from Azkaban's Dementors, which means that he must've felt the difference between level of protection from the Dementor offered by different Patronus variants at training session at Hogwarts, as well as the abnormal vulnerability to Dementors shielded by merely classical Patronus. The point of the training session now seems to be exactly the research on influence of Dementors on possessed Quirrell, with Harry's wand left near the Dementor possibly an experiment.)