While as you say, it can be edited out, I wonder how many nerds would be willing to do so just to take advantage of gambling bonuses?
It's not necessarily nerds, it's more people that want to make easy money and probably poor-ish people for whom like 100,- Euro is a fortune.
Abusing first deposit bonuses is quite a profitable business. Some example:
You have two bookmakers, that give you a 100% first deposit bonus, 100,- max. You register with both and deposit 100,- each. You now have 400,- in total. You look for some 2-way bet, maybe an over/under bet in American Sports, because odds are always good there and low juice.
Bookmaker A: You place 200,- on the over for 1.95 odds.
Bookmaker B: You place 200,- on the under for 1.95 odds.
This will leave you with 390,- in one of these bookmakers. But you still have to rollover your initial deposit. Now you go on to a third bookmaker, which also offers a 100% first deposit bonus and deposit 100,- there, getting 100,- on top. You again bet on some 2-way bet for 200,-/200,-. If you lose the bet at the third bookmaker, you will have 580,- (300,- of your own money in total) at the one bookmaker, with some of the rollover requirements already done. Then you move to the fourth bookmaker and so on and so on.
Now imagine, you can do that with a dozen of identities, this piles up to quite some "free of risk" amount of money and, depending on where you reside, some good income/wealth. And when you are able to make lets say 1,000,- with one identity, then people are willing to provide you with their identity, if you offer them 100,- for it. They can feed their kids with that money and don't care about possible consequences then.
And all this is the reason why KYC can only do so much to prevent multi-accounting.
If the main issue is with people using multi-accounts to abuse promotions, (...)
There are other circumstances (limitations for example), where you would be inclined to do multi-accounting. In these cases, you would use multiple accounts at one bookmaker only one after the other and not at the same time though. But I would think, that 98% of multi-accounting is done to abuse promotions/bonuses.
(...), then I would much rather use a casino with no promotions than complete KYC.
Nowadays it's very hard to get some market share, if you are not doing promotions. There is casinos and bookmakers everywhere and you have to lure people to come and play at your site. The easiest way to do that, is via promotions/bonuses. You will get a lot of abusers, but also lots of "honest" players. There will be talks about you on betting forums, you will be featured on some affiliate sites and in general get way more exposure.
It's an insanely contested market, just look around in this forum, that is not even a betting forum, and see how many gambling companies are advertising here and having signature campaigns.