It is either obvious that they should be entering only integers or positive numbers, in which case such feedback isn't helpful, or it might end up being too restrictive again. Who tells me that a system like this won't get used in order to force cities to have a population of an integer bigger than 10,000?
I understand the wish and desire to restrict user input, but I would like to remind everyone that Wikidata comes from the wiki side, which adheres more to the 'let's gather input and then verify it' than the 'let's make everyone give us correct input in the first place' side.