Some managers may not like illegal formation rule as it restricts creativity and flexibility on use of players. But it at least can help to punish for use of some weird formations that the match engine is not clever enough to simulate realistically. The 80%-20% procession was not an ideal design but it was a simple an implementation that, in my opinion, is make sense.

At present, I still have illegal formation in mind during team building & formation setting, except in friendly matches.