I don't know about that...most spotters don't know what they are doing I will say, but a good spotter is a great thing to have.
I am assuming most of these exercises you are hoping someone is doing in a power rack though, throwing 300lbs or more off your shoulders is serious shit with a bar especially if lifting at home. I like dumbbells for clearing reasons. You can sort of ride them down in almost any exercise you have a total failure on.
With the notable exception of the bench press, a spotter is not necessary for the majority of barbell exercises:
* Squat: should be done in a power rack with safety rails set-up just below full squat depth. If you get in trouble, just leave the bar on the rails. If you have bumper plates, you can also easily dump the weight. Spotting the squat is very difficult and most people do it wrong, which actually makes things more dangerous. If you train in a PL gym with trained spotters, they can be helpful, but that isn't the case for 99.9% of the population.
* Bench: as I said, this is the one exercise where a spotter is very helpful. Alternatives are a power rack with safety rails just below your chest, doing DB bench press, or leaving the collars off and letting the weights slide off (worst case scenario, but still better than being caught under the bar).
* OH press: if you can't finish a rep, just lower the weight back to your shoulders.
* Deadlift, rows, and most slow lifts from the ground: if you can't finish a rep, just set the bar back down.
* Any o-lift (clean, snatch, jerk): best done with bumper plates so you can dump the weight if you miss a rep. At any rate, there is *nothing* a spotter could do to help any of these exercises.
I do agree that dumbbells are great for some exercises (such as bench press), but they are very limiting on weight, especially for lower body movements.
