Now I definitely feel better about the strategy! Thanks for testing it out, SirPleb (something I haven't found the time to do, even though this has been percolating in my head for a while.)
What makes it more appealing in this case is the (assumed, since we don't really know yet) constrained land that is available to us. The only value the Worker give us after improving the Grassland, is to start working on the hills. It takes 6 turns to make a road, and 12 to mine, which only give us +1 shield/+1 commerce. Making a city on one of the hills give us +1food/+2commerce instantly, which seems a good trade off. And with increased commerce being desirable, its better to work a coastal space than even a mined/roaded hill, so you really shouldn't be worried about improving the hills. In a normal game the worker would go off and build roads, connect luxuries, irrigate plains, etc, which is valuable; that value doesn't seem to exist in this start.
For a general case, I think the best time to use the Joined Worker technique is a low food start (no food bonuses; a food bonus allows the standard start to grow fast enough to offset any benefit), at least 2 nearby BGs, with Industrious trait (Worker does more work before being joined) and/or Expansionistic trait (Scout looks around while capital builds fast Settler.) In this case you can end up with 2 cities and 2 Workers (and a few other units) about the time a standard start is spitting out it's first Settler. I've been tempted to quantify the pros and cons into an article; for a general start you have to delay your scouting because you're focused on building that first Settler (which is why Expansionistic trait helps). Since the benefit of joining the starting worker is usually marginal, it seems a poor tradeoff if scouting around is delayed and you can't make early contacts. (Not the case when you're all alone on an island; there's those assumptions again! ... only one more day...)