I think Opus17 covered all your questions. Generally, taxpayers carry over excess contributions because they no longer have enough money in the HSA to withdraw the entire excess (having spent it on medical bills). There is no upside to carrying the excess over, since, as Opus17 points out, you are going to have the excess added to Other Income no matter what, and if you carry excess over, you get dinged an additional 6% on top of that.
