I'm talking specifically about PT6A operations.
Once in beta in flight, you may not be able to come out. Once the prop starts to overspeed, the governor will try to control it by opening the bleed orfice, reducing engine RPM. As engine RPM is probably already at idle, it can't be reduced any farther, and the engine speed is no longer relevant because the slipstream is driving the propeller.
You're assuming you can get it out of beta or reverse, or that you'll have time. Chances are, you won't. When the prop starts to overspeed, you lose control...you'll be busy, and if you're close to the ground (when in ag are we not?), you probably won't get the chance to rectify the mistake. Even if you do, you'll likely be nose down, probably steeply.
When the prop starts into overspeed, it becomes self feeding. Adding power may only worsen it. You may not bring it out, you may just increase the Ng speed and increase the overspeed. Sometimes in an overspeed the only option is to go for feather, if you can feather it, and then come back out.
What does your airframe, engine, and propeller manufacturer say about using ground ranges in flight?
To address your other question...why not approach in flight idle rather than approach carrying power with the power lever? Simple. Your residual thrust is lower crossing the fence when you do come back to idle if you're already in ground idle. If you're in flight idle, your minimum Ng will be approximately 70%. If you're in ground idle, you can come back to approx 54% Ng, with less residual thrust, greater braking power from the propeller absorbing the airstream at idle, and reduced stopping distance.