I suspect the percentage would be pretty high. It's an easy and effective solution, but, it's a method frowned upon by SFDC (see a SFDC Product Manager, mtbclimber's response to this Idea).
Liked or not by SFDC, I just think people will continue to use it and they will thus feel obliged to support it. This is my favorite page to get the Field IDs for standard fields (custom fields are much easier - no need for a reference list):
I recently notice the list is missing one field from Case: Account. Even though it's automatically set when you select the Contact, you can use it independently if needed. So here's the Id to set it if you desire:
|CASE Field||Field Type||Field ID|
Info for the Parent Campaign field on Campaign.
Field Field Type Field ID
Parent Campaign ID Parent_lkid