Chris C. would be able to address this, but my guess the 10 percent illegal clue rule was probably a goodwill effort to keep from penalizing a contestant if their celebrity partner made a mistake, at a time where having that happen wouldn't clobber the budget. (It might have also been a slight nod to Pyramid, which probably had more celebrities fall into that trap--William Shatner being the obvious one, but I'm sure there were others we don't remember.)
Once the progressive jackpot was instituted, and the production company realized celebs could deliberately give an illegal clue with a second or two left in order to salvage a partial win, the rule obviously had to go.