To paraphrase: Why would I report gains on my investments.  I have a Swiss bank account.  The IRS has no way to know what I'm doing with my money.

You're correct.  It will be very difficult for the IRS to ever link that to you.  However, should you want to buy something with your BTC or convert your BTC into fiat, and the IRS links your purchase / trade back to you, then discovers you've avoided paying taxes on your BTC trades, you're looking at a tax evasion charge.