It's true. Here at Marketo we use Slack, and our Slackbot HATES server side post.. so much so that when you mention it, it chastises you. It's really pretty amusing.
Really though, Slackbot is completely right. Server side post has effectively been replaced by the following REST endpoint, which is great and scalable.
This, along with my other most recent blog, provide a good blueprint for associating leads in a non marketo form with only one REST call. No fewer than 3 customers have had this very question for me this week alone, so it seems like a hot topic.