I guess this one goes back to the old "pine body" argument: if it works just fine, why don't you see major guitar companies coming out with soft curly maple necks on their guitars? After all, the figure is much more plentiful in soft than in hard maple, so if it's an equally valuable wood for such a purpose, it would be used as often...right?
But how many guitar companies use soft curly maple for necks?
Conversely, how many use hard maple? More specifically, how many don't?
Obviously, companies make wood selections for many different reasons: tone, looks, stability/strength, price, and availability. Clearly, soft curly maple is superior to hard maple in looks (more common curl figure), price, and availability, so it's either tone or stability/strength that makes it unusable the companies. My guess? Both. I'm not trying to hinder innovation or "intimidate newbies into using their checkbook instead of their brains", as you so aptly put it, I'm just pointing out that the major companies with the custom shops running 24/7 have tried it, and rejected it, probably on its tone and looks, not some other reason, as is often the case with rare but tonally superior exotics.
Moreover, mistahJ, if the difference in price for a single neck blank of $4 or so is too much for you, you might want to consider reevaluating how much you're planning to spend building a custom guitar.
Now, as it has already been mentioned many times, laminating soft maple with a nice, hard laminate like ebony, wenge, bubinga, purpleheart, etc. will greatly improve the neck's strengh, as will adding CF rods. However, without these, the neck will likely be too weak, as you, Scott Rosenberger, and Westhemann have all pointed out.
Finally, why not use hard curly maple? Obviously CudBucket already bought his soft maple, so he can still use it, provided he adds laminates/ CF rods, and is prepared to take the risk that it may be more flexible than he likes. In the future, however, I see no advantage to purposely choosing soft maple over hard, besides the price difference, which is a rather small one in the world of luthiery.