Unnecessary medical treatments can have serious consequences. Even if you are absolutely certain you have a given condition, some treatments are very not worth it. More importantly, any given scan misses tons of important potential health issues. Not just the fact that CT scans don't show signs of MS vs MRIs, but neither of them will tell you about many cardiovascular conditions. So time spent treating made up issues from CT scans you don't need may be better spent doing other regular exams instead. Meanwhile, many aggressive cancers will be far too fast for any reasonable scan interval to catch in time. (And CT scans, to my understanding, can miss tumors early on, since it can't see as fine of detail as MRIs.)