function validate(val) 
   try:  
       cast val as numeric
   except:
      print "Oops, not a number"
      cast val as general  ## date or whatever 
end

validate(12.5)     ## returns 12.5
validate("12.5")   ## returns 12.5
validate("12 . 5") ## returns a date maybe

I mean, MS should change the default.

As a user, the only way currently is to make a template document and use that as a default when creating a new book.

You most certainly can. You can set the format of a cell, and if its set to number 12.5 will be 12.5, it wont even try date formats...