I believe the capital gains exclusion was originally intended to be fair, and fair to all, but it has been broadened so much it now frequently works a manifest injustice, esp. for the Romney and other Wall Street types.
There are many small businesses that require a lot of capital and work but throw off very little annual income. Farming is an example. I had a relative that farmed all his life, was extremely hard working and successful. Nearly every year of his farming life he took home, at best, near minimum wage as his compensation. In his seventies he retired, sold the farm and netted well into the seven figures. While he ran the farm he generated a lot of income and jobs for his community. To me that is an ideal situation for a reduced capital gains tax.
Where we run into problems is treating publically traded stocks, etc. as capital gains. There is no economic reason to do so. Neither society nor GM benefits at all if I make a profit on GM stock I've held, no matter how long I held it. And don't get me going on the BS "deferred compensation" Romney and the other fat cats ruthlessly exploit.
With substantial tuning the capital gains tax exclusion would be a limited but highly effective tax policy fostering economic growth.