Prot pallies could never be the MTs in a progression raid in vanilla; however, they were the kings of AOE tanking and frankly were better than warriors in this regard. They probably had the most reliable AOE threat generation. Things like the BRD emperor run really made prot pallies shine, as well as certain raid encounters. In fact this continued for some way into TBC until the AOE tanking abilities of the other classes were improved.
Having said the above, however, the vast majority of tanking in vanilla raids was single target tanking.