This answer is based on a conversation with Ryan Skinner, a Forrester Research analyst who specializes in business-to-consumer...
The registration wall is important for business-to-business marketers, because marketing automation systems can help drive the customer purchase journey. If users sign up to get past the registration wall, marketing can then do lead nurturing to accelerate, assist and facilitate that journey. Many of these registration wall forms are trying to assess what's called BANTs -- the budget, authority, needs and timeline of the customer -- so they can direct potential leads to sales or put them on a relevant nurture flow.
The question of whether to use a content registration wall requires that marketers confront a tradeoff. Putting content behind a registration wall means that it won't be crawled by Google, so people are less likely find it. The form itself is likely to cost potential viewers, too. Are you willing to lose 50% of the audience, or maybe more, if the registration form is particularly onerous? Take away that form and a lot more people will likely see that content, but you don't get the lead data that you want.
One general principal is that if you're trying to influence people, you want to go for reach. For example, if you've written an article advising readers to rethink how they purchase ERP software, where the goal there is changing hearts and minds, you want people to see it. So, losing 50% of the potential audience is unacceptable there. Another example is early awareness content, where you're trying to drive awareness about your brand or a change in mindset -- you wouldn't want to lose a significant portion of the audience there by forcing them to sign up.
A registration wall is a better fit for mid-sales-funnel considerations, where the goal is trying to inform and influence the buyer's activity and strategy for making the purchase. For example, you could offer a guide for working with key stakeholders during a technology purchase, in return for registration. In that case, you're offering something that is useful for the users, so you can ask for some information in return.
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