The big sponge filter is on my 10 gallon tank and the long tube is almost out of the water. I was wondering if I really need to have that long tube attached. also where does one put an airstone on these sponges? thanks again 

I think the long tube contains the bubbles so it helps to establish water flow through the sponge. But, I know mg has at least a few tanks where he doesn't bother with the tubes so who knows. My sponge filter has a little nub that you attach the airline tubing directly to the plastic, no airstone required.
You can trim it a bit.
Having an airstone increases the fraction of the tube's cross section that's air bubbles - tube length aids in generating even more of an air bubble/water ratio as the bubbles expand as they rise. Lots of air travelling quickly up the tube means more water movement up the tube, which means better filtration.
If you don't want to become an advanced student of fluid dynamics while considering further mods to your sponge filter setup, just fit it w/ a powerhead. They're Tons more efficient.