I don't get it either, but it is what it is.

The BJ arm count is no different than 5-4-3-2-1 ticking off of a play clock, IMO...and in no way an advantage for either team. Team B could just as readily have an assistant counting it down verbally from the sideline to the defense if they felt it was such.

Rich...how long 'til we get the 5 count pulled in WI ?? Over/under is 2 years, I'll say.

Not going to change unless the NFHS mechanics change.

This is what I found from PIAA:
Back Judge
Always positioned deeper than the deepest defensive back, normally taking an original position no closer than 15 yds deep and no more than 20 yards deep.
When the ball is on an inbounds line (hashmark), position yourself no wider than the opposite upright.
Know your eligible receiver responsibilities.
Echo referee's down signal verbally and with an overhead hand signal back to the Line Judge and the Linesman.
Responsible for the 25 second count. The PIAA mechanic is to raise and hold one arm straight overhead if the team has not snapped the ball at 20 seconds after the ready have elapsed; lower your arm at the snap or when the team has committed delay of the game.
If the field clock malfunctions, you will keep the game time and the referee will keep the 25 second count.
Responsible for counting defensive team. Confirm count with Line Judge using an extended fist; once in agreement, drop hand. Observe that replaced players leave the field immediately.
Key the tight end on the strong side. If a balanced formation, key the end to the Line Judge's side. On runs on either side of field, move to a position so that the runner is between you and the Linesman/Line Judge, observing blocks ahead of runner.
On out of bounds plays. especially in the team areas, move into the area of the dead ball and assist other officials in maintaining order.
With the Line Judge, echo the Referee's signal (winding the clock) when he starts the clock
The Back Judge has responsibility for the end line.

Source

In WA the BJ for crew of 5 and the R for crew of 4 (only the highest varsity level gets a crew of 5 so we do a lot of 4-man varsity games) keeps the 25 second clock and raises one arm above head when the count gets to 20.

Only 2 of the 14 fields we use have a play clock on the field.

MN does the Back Judge basketball style count as well.

At one point, I think there was a suggestion that the R give a verbal count of the last 5 seconds at the same time... but that was quickly rejected because if the R doesn't see the B start the count, he could be counting "5...4...3... (B whistle... flag)", leading to the inevitable "But the referee was a 3 when the flag came!" fiasco. Plus, the R shouldn't be looking at B when the snap is imminent anyway, R should have a lot of other things to worry about.

Seems weird that IA wouldn't want a visible count, but I'm sure the Powers That Be(TM) in IA have their reasons.