You have a very limited number of dollars to place into this awesome, semi-magical account which offers tax-free compounding, tax-free dividends/interest, and tax-free capital gains. That means any dollars you are able to get into your Roth are more valuable to you than the same $ in a taxable acct. To illustrate, consider if you had a choice about where to lose $10k: in your taxable account where the loss would offset some of your income tax or in the Roth where, if you max it out every year, the lost funds can never be re-deposited and where the funds would otherwise have grown tax-free. Obviously the Roth dollars are worth more to you.
This is an argument for putting your risky investments in taxable, or at least pre-tax. That way if they fail, you lose the less valuable $.