The promos and challanges are like a "Faucet 2.0". Basically it serves its purpose like a faucet however it filters out most of the people who will abuse the service.
The issue with the faucet was that there were too many people who weren't really gamblers, they never made a single deposit to the site and were just in it for the free money by risk-free gambling.
By having these promos it lets people who are actually interested in gambling and loyal to the website to get some type of "faucet" funds they can gamble with. So this filters out many of the people who would abuse the faucet without actually making a deposit.
Exactly, they basically made another faucet but called it not faucet so the bots and abusers would stay away, it requires a bit of work but nothing major and it is still quite easy to get free money from them, all you have to do is spend a bit of time. Hell, if I am not wrong they are giving away free money to people who post on their own forums as well, so you are still getting paid for basically writing (kinda like signature campaign) so it is not risking any money at all, it is not even risking too much time neither, just stops abusers and that's it, still gives people free faucets.The issue with the faucet was that there were too many people who weren't really gamblers, they never made a single deposit to the site and were just in it for the free money by risk-free gambling.
By having these promos it lets people who are actually interested in gambling and loyal to the website to get some type of "faucet" funds they can gamble with. So this filters out many of the people who would abuse the faucet without actually making a deposit.
Of course, there must be some people who try to abuse even that, we can never have zero abusers but at least it could stop the most common ones and make the current ones work harder for it.
Yes it's still possible to have a few unethical people who will abuse the promos but it's very difficult and more human interaction is required so most faucet abusers will just quit.
The reason why the faucet abuse went on for so long is due to the extremely cheap prices for solving captcha's. Most casinos assumed that Googles captcha is pretty difficult to cheat, especially when you need to select buses or street signs instead of just typing letters. What they didn't realize is that there are services where there are people who are paid like $1/hr to solve captcha's non stop. They use this service along with some API bot and take a bet on 99x and eventually they will reach the min withdraw and repeat the process.
I think PD was losing like 20BTC a day in the past due to the abuse as far as I can recall. Hence this strategy is more bullet proof.
) All in all I would say raffle would have been expected by me, I have been that much lucky with primedice so not being there is a total shock 
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