A free play is worth roughly half of cash for the avg bettor because if you wager it you end up with cash as a result and the free play amount is gone. What is left is cash from the wager if it wins. A cash bonus is just like a deposit credit but it comes with requirements such as betting the deposit+bonus 4x. Bodog is the only 1x bonus right now. Freeplays also have play-through requirements.
Yep. Say you get a $100 freeplay and you wager it on a -200 favorite. If the favorite wins you end up with $50 credited to your account that you must "rollover" (bet) a couple of times and then you can withdraw it for cash
As everyone suggest, using a free-play on your dog picks is the way to go, however, if you do not have a balance and you just received a free play and that is your only balance, then of course you would want to use it for a favorite or whoever you think has the best chance to win the bet so that you could turn that free play to cash.