I assume that the robots will not worry about the registration process. Your average simple-minded robot is simply unpacked into a form that does not require any other actions (authentication, identification). The simple action of requesting one or more additional clicks will prevent most simple “attacks”. If you look at the blog site for Coding Horror, they use captcha with a permanent post word.
On the other hand, while a few extra clicks will hold the silent robots, they will not hold human spammers, pranksters, grippers, etc. But then again, the extra email addresses are pretty simple, so if someone really wants to fill your site with junk that they can.
My conclusion is this: I think you will get from 10% to 20% more "garbage" on your pages and 5% and 25% more "desired" calls, depending on how much it bothered your potential customers, Thus I do not see much harm in removing the email barrier.
Carl Smotricz
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