No, it's not logical. I would call it 'letting the perfect be the enemy of the good.' Any information entered in a field might be incorrect, and blank is the most incorrect of all. If you are comfortable with others estimating a parent's location based on his children's profiles, why would you shy away from making the same estimate yourself on his profile?
It's OK to be incorrect, on a good-faith basis. What do we ever really 'know?' Unless you have a primary source for a location, then anything you enter could be incorrect. Even primary sources sometimes have errors. Blank is incorrect always.
On the other hand, if you know where a person's children were born, you know the person lived in that place at that time. Where else would you start looking for him or her? That's the best first approximation you have for the person's birth or death location (pick one), and you should put it on the profile: 'Europe,' 'Central Europe,' 'Holy Roman Empire,' 'Silesia.' Something vague is better than nothing. Click the 'uncertain' button and add all the disclaimers to the bio.
Even if it's off by hundreds of miles, you have done your fellow members a great service. The next time a member looks at a list of potential matches for that name, he or she can tell at a glance whether that person merits further investigation, just from the info presented in the list. If the location fields are blank, you deny that member the option. How collaborative is that?