I have a question. What are the negative consequences of giving the untimed down signal on a try?
Other than ridicule by those that know better, nothing, really. But, then, what are the negative consequences of using the illegal procedure signal for offside? The offside signal for offensive encroachment? Dropping a bean bag at the spot of a muff by Team B on a free kick (onside, typically)? Using a chop AND a wind to mark the ball ready for play and start the game clock? It may not affect the game, and almost no one except officials will know the difference, but wrong, nonetheless.
The signal first appeared in 1994 (not as long ago as as I thought). As I said before, it was put in to help pressbox folks know what's going on when there is to be a play in an extended period. They may wonder why the teams aren't moving to the other end of the field. This signal will give them the reason, if they know the signal, or bother to look it up in the chart.
Anybody that is confused about what is happening when there is to be a try just needs to get out their make-up bag and freshen their lipstick.