Ya, not allowed, though... no links to our own products or email addresses. The idea is you build reputation by solving people's probs, and that in turn can make people seek you out.
On a technical level, you should consider that your plugin (like Grazitti's, and any other server-side plugin like this that I'm aware of) consumes a REST call for each anonymous session, so anyone can trivially break it for the rest of the day -- and break all other integrations across the Marketo instance -- by trickling in at most 1/2 hour of traffic.
In 2018, everyone must expect attacks on their corporate website; everyone is a target, not just "controversial" sites. Sure, at some scale, a DoS vulnerability will always exist. But that scale shouldn't be a single bad actor with a few minutes to spare. That's why I don't consider these approaches prime-time-ready.
This isn't to say, by the way, that your plugin isn't superior to the other REST-based solutions! But REST isn't the way to do this. Any solution must have as its baseline the same capacity as native Marketo PreFill.
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