People who call it cancer tell only half of the story.
Let's take Warsong, a Feenix vanilla realm with x12 xp rate. It also had a vote shop, which means you could get various blue quality items for vote points. These items were often exactly the ones you need to prepare for raiding, like Scepter of Interminable Focus. To understand why GDKP was a great system on that realm we have to dig up a bit of history.
Warsong ran for a long time (5? years) and had Naxx progression .
Naxx had been out for ages, with bugged end boss Kel'Thuzad.
Obviously this means all hardcore players had raided Naxx , had all their gear, had quit the game altogether, replaced with the next generation of hardcore raiders, who raided Naxx, had all their gear (well 8/9, as again end boss was bugged) you get the point.. a very mature realm. With tons of gold in circulation.
Established guilds would only do AQ40 for Cthun and Naxx. Old fashioned guild MC runs were dying out. Even new guilds could skip it, do some ZG, get Preraid items from voting and jump directly into BWL-AQ
Enter the 'invention' of GDKP.
Hey!, My 8th alt (x12 xp remember, you have a lvl 60 in a day) can now just get starting MC or even BWL, bring 10,000 gold and buy some items? Great!
Everybody wins.
The few tryhards that wanted their 2nd Binding to drop and organized all these runs won.
The new players who entered with 500g and came out with 1760g (but no gear) won.
The rich alts who entered with 10k and came out with 4K and their needed Talisman of Ephemeral Power won.
I came late on Warsong, because I played more on the other feenix realms. I was able to earn around 12000 gold and deck out a rogue and a mage in ZG-MC- AQ20 lvl gear within a month.
There would be several long running GDKP organizers who would post their raids and the results on the forum each week. If you chose one of those you would never get scammed.
But it killed regular raiding?
No it didn't but the frontline of raiding had moved to AQ and Naxx.. It's the same reason we don't do UBRS raids with a guild anymore...
But the new guy can never buy anything . That is not fair!
The new guy has to work for a bit, but within a few raids, he will be loaded with cash to start buying some items for himself. In a guild run you don't get the A-class items that veteran members still need either... You have to start at the bottom, but actually GDKP for pugs would get you a lot of gear in a very short time. that is why it was so popular!
But I never will get Nefarian's Tear this way because Xmas will buy it for 50.000 gold!
So? Aren't you happy that that big chunk of gold is divided by 40, back into the community again? Long live AH junkies and their fat wallets.
But I would rather roll for my items!
Make your own pug raid and see how many people you attract... oh you didn't find a main tank, only 13 rogues and 7 hunters.... maybe RNG is not that popular as a loot mechanism after all...
People want to raid, and raiding at the start of World of Warcraft had a stupid amount of RNG build in.
The players didn't like it from the start. The complained. Blizzard responded. By the 3rd raid instance (Zul Gurrub) you already can see the changes. Instead of <bracers of druid> that get disenchanted immediately, we now get <quest item of Druid, Rogue and Hunter> A great improvement!
Players made their own improvements within guilds, with RPP, DKP, later evolving into, screw this bureaucracy, we are friends, we can decide among ourselves who gets what. Loot Council.
But you are still bound by the guild system. For various reasons players started to some 'side-raiding' in pugs as well. But they don't to go back to the stone age of raiding with maximal randomness. GDKP is, in certain conditions, a great system to minimize the random factor in entry level raids.