I don't know about Yahoo, but I get into Hotmail OK with read-only cookies.
I am going to see if I can get organized enough to have more than one cookie file so I can switch them around... dunno why, except I'm naturally sneaky
Been sutfing for a week with "unprotected" cookie file, I forget to change it! Must weigh a pound by now.
I had a problem with a program once, it wouldn't keep my settings - so I made its ini file read-only. Yepper! nice.