You probably have a slipping clutch band in your transmission. Take it to a transmission shop.
Any unnecessary shifts cause additional wear on the shift mechanism, but that's a minor consideration. At a long signal light or train crossing, it's better for the transmission to shift into neutral than to keep it in drive with foot on the brake, or if a manual, better in neutral than holding in the clutch all that time.
The transmission "valve body" can often have problems that result in leaks and sticking. Once the seals of a valve body start to fail, the transmission clutches start to slip and eventually fail altogether. In any case, once the transmission fails in the way you describe, you're pretty much stuck with the only options being to rebuild the transmission, replace the transmission or get rid of the vehicle.
A hard shift isn't always hard on a transmission. When a transmission shifts soft, it has to slip clutches and bands which causes the friction material to wear out faster. But if this concerns you, have the transmission vacuum modulator valve and vacuum line inspected as this controls the shift firmness.
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