I have already completed all my core courses, and more than minimum requirements (bunch of courses in Operations Research and Transportation Systems). However, it always seems to me that there is just more stuff I want to keep learning (mostly in Applied Math). I usually don't take more than one or two so it doesn't affect my research time. When and Why did you stop? Did your advisor tell you to stop? (my advisor doesn't really mind, I once mentioned to him about getting a Master's in Applied Math, and he said that's fine with him). I have two Master's degrees (Transportation Engineering and Applied Economics), and I am currently finishing the dissertation for my PhD (in Economics) requirements. I already passed all qualification exams and prospectus defense. Also, there's more than enough funding for me (I am funded by my advisor's chair funds).