[chroem-]

I don't think you would even necessarily need stages if you don't have any empty fuel tanks to discard. The engines might not be as optimized for operating both in and out of atmosphere, but that seems like a fair tradeoff.

[Symmetry]

Part of the reason for staging is the tanks but part of it is the weight of the rocket engines themselves. The original Atlas had a single tank that started off with two large and one small engines attached to it. When it had been lofted enough the two large engines dropped off and the small engine finished pushing it into orbit.

[outworlder]

Yeah, tanks are not THAT heavy. But you are forced to use multiple engine types due to the bell nozzle.

If we used aerospikes, then it would be a different matter.