Memory card file delete or not?

I am doing digital photography since last 7 yrs. I have Nikon , Canon Dslr systems,. I used to edit , delete files and make new folder on memory cards itself before copying pictures to hard disk. I never encountered problem,

Recently ,I read that deletion editing shud not be done on memory cards. It can corrupt origina files or memory cards. I want to know what is ground reality. Has any body bitter experience of getting card corrupted or this is just safety precaution. Particulary, what is harm in seeing files direct from card and deleting it from there?before copying to system.
Pl tell ur experience not theory?