Users could add their own profile questions/fields, and you could have a “click here” to add this question to your profile button.
Allows for massive profile flexibility. User-driven (users determine what the best profile fields are).
It would be slightly slow from a database perspective (as the extra fields would be stored in a people-extra-fields table).
Searching on the extra fields would be difficult.
Getting users to define an extra field would be messy. Fields would probably be limited to strings. I could have a default size (eg 255 varchar), but users would still need to come up with a good field label.
Advantage - this could be used by administrators to expand the number of fields, and to customize the software for their own network.