I've argued that same concept during the frequent 1/1 arguments that show up on these boards. It is true that every single serial numbered card is unique to all of the others by nature of that serial number. There may be 2,500 of the same looking card, but each one IS DIFFERENT.
UD took that to a new level when they issued 99 Steve Garvey cards a few years back, all numbered 1/1 on the front. The only difference between the cards was an achievement shown on the front of the card (34 triples in 1977, .345 BA in 1988, 3rd ASG in 1981). The design, picture, etc were all the same otherwise. I felt those were about as 1/1 as a serial numbered card with 99 copies and the prices for them showed this too, although not THAT cheap. I got numerous in the $10-20 range though.