Related gripe: Requiring people to sign-up--or worse, to already be a customer--just to view API documentation.
Y'all're crazy if you think your API is so awesome that it needs to be a trade secret, and without it I can't get a good idea if you product is something that would actually solve our problems, or whether it seems like something worth integrating-with.
A major change in my field over the last few years is the rise of Ignition, a SCADA suite that's taking over everywhere. And sure, it's got simpler and cheaper licensing than it's rivals. But for our projects, licensing cost isn't generally a factor.
What Inductive Automation did was open up their documentation, offer good online training for free, have an endless demo mode that can be reset indefinitely, and a "maker" version that can be used for free. Oh, and it's scripted with Python instead of some janky BASIC knock-off. All features that appeal to integrators but that don't matter to the end users.
Putting these walls up also makes life harder for actual customers, who now have to prove they are such every time they need to access said docs for whatever reason.
As an individual engineer, if I have to jump through hoops just to login and view docs... yeah. I won't be going out of my way to make sure you get considered in trade studies at my next job.
In my experience, non developer audiences like demos. Developers tend to like to try things out on their own, maybe with a little tech support.