I have done this.
As an example, I have a quote system that loads a list of permitted product descriptions from an external system. If they want to change the description, originally they had to enter it into the external system, and re-export it.
we changed that, so they had a combo box of existing descriptions to guide them, but limit to list was set to no, so they could actually enter whatever description they wanted.
you gain some benefits, and lose some benefits by having free text fields, though.