Phil, to me one of the beauties of the current system is that it doesn't even require a volunteer, just 2 players following a simple procedure. And I personally like the idea of the system remaining the same throughout the ranks, rather than requiring a player to learn a new system as he progresses to an event with spectators and a full computerized and televised scoring system.
A tennis court is tiny in comparison to a golf course. I really don't think there is a valid comparison out there to another sport (if there is, I'd ask someone else to point it out, we may all learn something). Tennis requires continual updating of the scoring, as each game/set/match is based on it. That's where Shivas's analogy to the Federer/Nadal match fails, not because the players are too tired to keep score themselves and sign cards afterwards.
Many of this site have a (healthy) resistance to change. I've had the good fortune of spending a good bit of time with JohnV and I can tell you, he's no Luddite who abhors change. He simply wants to get a tournament done in the best way possible. I haven't had the pleasure of spending much time with Sully, but his experiences ring true to me.
In the end, you are talking about changing a rather large system to accommodate someone simply forgetting to sign a scorecard - that seems like overkill to me. I wouldn't be against trying Shivas's system out on a small scale someone to see how it functions, but it just strikes me as overcomplicating a rather simple task. I look at it this way: If I were running a tournament, which system would I choose, if the rulesmakers didn't stipulate one? The current setup appears to me to be both simpler and have less potential for problems. I haven't seen it in action, but having 2 sets of scores on each scorecard and having someone else compare sets of 2 scorecards, checking for agreement, seems like it has the potential for more problems. (Most of the other recommendations from others seem no more reasonable to me, btw.)
And that's all said without even referencing tradition, personal responsibility, etc (things which also resonate with me, but are left out for purposes of reasoned discussion of the task of scorekeeping in golf).