Allow his interest to cool off for one second and you run the risk of him getting distracted and forgetting about you. Or, worse, getting frustrated. So, if you're going to ask him to share his personal data, he'll expect you to cough up the goods, tout suite. And they better be good.
But why force him to jump through hoops at all? A problem with the "tell me and then I'll tell you" strategy is that it puts a speed bump in the road to building a relationship. Sure, it's not an insurmountable bump, but anything that gets in the way of engaging the attention of a potential customer is a bad idea.
As a vendor you need to be as open to your market as possible. Particularly in the enterprise market, secrecy through obscurity is outdated, which is why so many companies are going open source ... the more people who know what you do the more understandable your value proposition becomes.
So, vendors: What's your rationale for trying to keep stuff secret? And buyers: How much does vendors being coquettish about what they do annoy you?
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