The real reason there are a lot of uphill par-5 holes [I think] is to reduce the NUMBER of uphill holes out of 18 in the routing. You've got a better shot at making a good uphill par-5 than you do of making a good uphill par-4 plus a good uphill par-3. [It's the flip side reasoning of why there are few par-5 holes along the oceanfront ... because in that case you'd normally opt for two holes instead of one.]
I had never thought of Patrick's point about the uphill grade defending par on the longer holes. I guess it's true, although you could say the same thing for a long par-3 or long par-4 that plays uphill, that the architect wanted to defend the strength of those holes.
I do concur with all of those who cited the 8th hole at Crystal Downs, or the 4th at Bethpage Black. Those are two of the great par-5 holes I know.
One of the best par-5 holes I've built is the 4th hole at Tumble Creek, and I can tell you the hole was conceived entirely for the purpose of getting up a big climb in one swoop.