To be fair, getting to pay no taxes on something that you sell for a big profit (your house) is a little bit ridiculous, IMO, anyway. Nobody would make this argument for, say, a car, or a gold coin, or a stock/bond, or a collectible.
I think the old system where the proceeds were tax free only if they were used on your next house made more sense, really.
The bottom line is that both the capital gains exemption and the mortgage interest deduction are pretty indefensible - from both sides of the political spectrum. IMO it would be best to just get rid of both of them cold turkey. Cheaper housing would be a net positive for the economy.
-W