I do not know why so many discussions end up this way, you get more vitamin C from an orange, but more beta carotene from a carrot. Some applications fit processor A better than processor B and some fit B better than A. Would you run your assembler on a mac, would you do graphic arts on a PC? I see no reason to discuss whether a PIC or a '86 or a thingamajig is better than a '51, this is application dependent.
The real issue is that sometimes you put an application into a choice which may not be the best in order to save the investment in development tools and knowledge gathering.
The advantage, if any, of the '51 is not that it is a better processor (sometimes it is, sometimes it aint) but that it is multisourced with almost any combination of peripherals that you can imagine.