OK...without getting into "when is a web developer also an actual engineer?" debate...what do these engineers do when the marketers don't have any big ideas at the moment? Engineers on an engineering team ostensibly always have something to work and iterate on, directly...but if they're at the behest to the marketing team, then that's not a particularly agile process.
Well, I mean it could be, but then you've set up a shadow engineering team that is basically staked with creating whizbang ideas to compete with what the non-marketing-engineers are toiling to maintain. Actually, if I were in the non-marketing team, I'd find that quite annoying, as the old-cash-cow is always more annoying than what the bleeding-edge dev team is working on. But this is even worse than the Apple ][ team competing with the Macintosh team...the marketing-engineering team, in the OP's description, could really go down the road of creating plausible vaporware, sucking the attention of the company management.
The OP does give a good real-life example of what the marketing-engineers can do: a documentation portal for API developers, and visualization tools...two things that could be from the main engineering department but could also be completely separate...yet require key technical skills. But in that case, it's often just kind of another engineering team, at a mid-to-large-sized company that may have several teams (like how Google has search, local, social, whatever Peter Norvig works on, etc).
I guess I just don't see many situations in which building out such a parallel engineering team would be feasible for most startups, where the duplication of certain infrastructure and procedures is far more costly than it is for an established company.