As a Moodle site administrator and user, I care about the database implementation only because it affects performance. Other than that, I'm a lot more interested in how this will be implemented in the UI.
We are a Remote Learner customer, and we have ELIS. We find it very difficult to use, primarily because the added fields are not integrated with the objects they go with (other than User Profile fields). For example, a field added to a course attaches to a separate "Course Instance" object instead of attaching to the actual course, and the fields are not visible within the course itself. This means we can't use these extra fields to store information about each course in a way that other users, e.g. instructors, can easily see it. We can't use existing bulk course creation tools to modify these fields. (Remote Learner's own Integration Point/DataHub utilities require us to update Moodle courses and ELIS course instances separately.) We can't report on those fields easily, either.
I was very excited to see the integration request by François Gannaz in MDL-34634 because it seemed to me that this would integrate with the existing functionality in the same way that the User Profile fields do, which is very usable. Couldn't we get this moving for inclusion in 2.6? Are any of the other proposed back-end implementations that much more efficient? Are we letting perfect be the enemy of good?