I had a Tahiti with log style exhaust never had a damn Gate Valve
Had pump to Tee into motor out T-Stat housing into & out log exhaust no throttle valve at all. that motor lasted till last year it was 35 years old
My buddy had a Southwind with log exhaust with a gate valve off the pump but it was wide open
I agree if would save you if you were sinking (Or stay outta shallow water and dont run over your ski rope) but where the hell are you guys driving your boats through all this sand and shit u speak of. I'm in lakes with relatively warm water and not driving over shallow stuff or rocks.
and a gate is not the best tool to control water pressure thats all I will say and I'll leave it alone.
I'm gonna start making gate valves and selling them to you guys 
Sand and silt is a very real and big problem when you run a jet boat where they are designed to run. Any fast moving water has a ton of free flowing, very fine sand, and as long as the engine is running, acts like a huge vacuum and sucks up tons of the loose material.
If you make a gate valve made in the USA for a reasonable price that has a way to lock the position and has indexing marks for percent open, I would be all over it...

And just for reference, not all jet pumps produce the same amount of flow and pressure, the amount of HP typically determines how much pressure and volume they are capable of, but sometimes it is the pump itself.
Not trying to start a pissing war, just trying to keep the facts floating on the top.
Also, the fact that you had a boat run for X number of years without a valve don't mean Squanto. I call it blind luck. JMHO
GT