I certainly agree with the general principle that too little resistance is not good for a spring piston (gas or metal spring) gun because the gun depends on the compressed air to cushion the piston as it slams the end of the chamber.
That is the rationale behind using more than one cleaning pellet since a single cleaning pellet doesn't provide enough resistance. I have a couple of cleaning pellet brands on hand. RWS states that they don't really recommend using the shoot-through method but that if it is employed more than one pellet should be used. They primarily recommend using the cleaning pellets as an alternative to patches--pushing them through the bore with a rod.
The material with the Beeman pellets recommends 2 pellets for pistols and 3 for rifles when using the shoot-through method. Their material doesn't mention using the pellets for push-through cleaning.
Just out of curiosity, I weighed some cleaning pellets. It turns out they have an average weight of 0.7 grains. That's much lighter than I expected.
That probably doesn't tell the whole story because it's been my experience that they are significantly harder to load into the bore than a typical lead pellet. So a little bit of what they lack in weight they probably make up for in bore resistance. I don't have a good way to measure bore resistance, so that's mostly speculation.
It is possible to add weight by oil soaking one or more of the pellets. An oil-soaked .177 cleaning pellet weighs about 2.4 grains--now 3 of them will weigh 7.2 grains and 2 will weigh 4.8 grains. That's in the right general neighborhood (albeit a little bit light in the case of the two cleaning pellets) in terms of pellet weight for a .177 gun.
I think you're probably on the right track in terms of not recommending the pellets (even with multiple pellets loaded) for any kind of routine shooting. Especially if they are shot dry, they're a lot lighter than a typical pellet and will stress the gun.
However, for an occasional quick & dirty cleaning job, I wouldn't feel badly about using multiple cleaning pellets at once (especially with one or more of the pellets oil soaked) with the shoot-through technique.
I'm glad this came up. I hadn't ever bothered weighing the cleaning pellets before. I don't think I'll ever shoot them dry again, regardless of what the enclosed material suggests.
Ideally you should (imo) put one lead pellet in backwards, then the cleaning pellet, followed by another lead pellet.
This should create tremendously more resistance/friction than a normal pellet. It is my understanding that too much bore resistance in a metal spring gun can cause a more abrupt piston rebound than normal which may eventually kink or break the spring--sort of the same kind of damage that dieseling can cause since it's the same basic mechanical cause. It should not be an issue in a gas spring/nitro piston gun.