Why do you think the exchange rate of USD abroad is worse than the one given by your bank? IMHO, you don't need to be very smart, walking in Moscow, to see that with 1U$S you get between 56 and 57RUR in any bank (that would be charged to your credit card), but 59RUR in the street. So there is a small profit (some 5%). It's not much but it helps.
Again, there is a risk with carrying cash, but so is a risk to having a debit card. Say you loose it, or it is stolen. How easy it is to replace it? Is it even possible? (In Argentine banks they only issue debit cards in the bank, not abroad, and they don't send them to you by post, as you need to receive another envelope with the PIN Code. Maybe in the US or in Western Europe the technology is better, but I was stolen my wallet in Athens some years ago, and I had no way to get a debit card back. Also was stolen my drivers license, so I had to cancel all the car rentals I had booked in Europe. Traveling with cash (cash was not in the wallet), I ventured on an epic journey with the Balkan Rail Pass, that would eventually open my mind to the Balkans, to Eastern Europe, and finally to take away my fear of colder Cyrillic countries, and some 3 years later I'd be living in Russia....