And pretty much anyone will consider David a horrible failure that he hasn't even made a GS final since that surprise Wimbledon run when he was a young kid. He hasn't even gotten to the final since, and hasn't made too many deep runs either. People won't remember David as a great player, they will remember him as a great talent and someone who did possible the least with his talent than anyone ever in history.
That doesn't change the fact that Kolya hasn't made a final. It's a sin that Nalbandian doesn't have a few slams, but it's still better than semis. They are more or less equal further down the line, so unless the famous Roddick argument of years in top 10 or top 5 is invoked, Nalby has accomplished more, however inconsistent he might be.