Shadow, consider this.
Imagine that jetbikes had a 20 guardian tax at 28 pts per guardian (Equally stupid I know).
That would be a 560 pt tax on your jetbikes. Even in a 1500 pt game you could, if inclined, put out 120 S6 shots a turn.....120 with a tax as stupid as 28 pt guardians.....thats a bit beyond under priced mate.
Edit: Now your right, they'll suffer from diminishing returns.
But that 40, thats a single 10 man squad, split that in 3 (for your FA slots) and yout still putting out 40 S6 shots a turn, you're just doing it even more efficiently
What I suspect we'll actually see is squads of say 5 jetbikes, puting out 20 shots each a turn, giving the army as a whole 60 S6 shots for only 405 pts. Plenty of room for D-weapons, Seer support (or stars if they are still a thing) etc.