I don't see any point in cutting it. As long as you keep a barrier between the aluminum and steel to prevent corrosion, you should be fine.

But it brings to mind an interesting, if possibly meaningless, question. Say the frame is double butted 531 or similar. How far down the seat tube does the butted section of the tube extend? If the extra long seat post protrudes down into the un-butted, larger diameter section of the tube, are there any implications with regard to leverage, lack of contact with the steel, etc.? Probably not. I'm just letting my fetid imagination wander, as usual.