Running a sponge filter is not the same as having an airstone in each tank. Even though you will have the same output of air, it's going to have a different effect on CO2 levels and so forth. Even just going from finer bubbles to coarser bubbles from using a diffuser vs. an airstone, gives you a slightly different result.
However, if you're going to bother with running an air pump anyway, the way I see it you might as well be using a sponge filter...it will be vastly more useful in a tank than a naked airstone. Fish do like airstones, they play in them - but they aren't necessary. They may be slightly beneficial if your tank is sufficiently filtered with adequate circulation, but mostly in that circumstance they are a backup in case your main source of filtration fails. A better backup is a sponge filter, not to mention it provides a source of seeded filtration in case you need it!
Going from mechanical filtration with an airstone to only air powered filtration will probably not make a visibly positive effect. Mechanical filtration does polish the water better, in a tank without plants. Successful, beautiful tanks powered solely by air powered filtration rely a lot on water changes and plants, in my opinion. Sponge filters tend to get funky very fast if they are the sole filtration in a tank that is stocked normally and not jammed with plants.
