I definately have to disagree here!!!! EVERY professional guitar company on earth builds their maple necks from hard maple. Except for custom builders there are no exceptions I know of. Most books on guitar building recommend to use hard maple. I myself buy from the owner of the company supplying Peavey, Fender, etc. with neck woods. This supplier sold all neck blanks for the Peavey Wolfgang Series. I asked that guy exactly this question and he told me not to use soft maple because of several reasons:
1. It is not stable enough for a neck if not quatersawn or laminated with stronger woods and tends to warp
2. It gives you a neck that is very easy to bend
3. A soft maple neck gives the guitar a quite mellow and dull sound (this may be desired in some scenarios)
I myself choose to build the neck for my 7-string from quatersawn soft maple laminated with bubinga anyway, because I wanted the curly figure. While the neck is stable enough due to the laminates and the soft maple being quatersawn it sounds way to mellow for my taste. I will build a new one from hard maple.
So the bottom line in my opinion is: You definately can build necks out of soft maple, but it is only suitable for a certain sound and certain construction techniques which make up for the soft maple's weakness. Describing soft maple as the way to go when 98% of all guitar necks are made from hard maple with good reason is quite ridiculous....