My recollection of the reason for ditto is slightly different.
When Rush first went national, many callers would start their comments by saying something to the effect of "Rush, I've been a conservative for many years, and I found a breath of fresh air on your radio show. My family and I love your show. . ."
After a few dozen of those introductory comments, a caller said "Ditto" to the previous caller's introduction and went on to the real reason for his call. Then the rest of the callers starting saying "Ditto" to avoid syrupy introductions.
Since many of the Dittoheads call to disagree with Rush, Ditto seems more of a shorthand for "I love your show" than a code for "I agree with Rush."
I wonder if other Freepers have a different take on this.
There's a local talk-radio host here in Houston named Chris Baker who does a similar thing. Instead of wasting airtime with a lot of stuff like "Hi, Chris-how ya doin'", and "Fine, how 'bout yourself?", he asks that callers say "Yahoo, Chris", and he says "Back at ya", and so on. Smae thing as the dittoes, basically.
This is my understanding.