Ahlony Wong, this 'illegal formation' thing is something that I have never agreed with in this game. Like you said, you should be free to use whatever formation you want, that is there should be no penalty in these terms. However, if ever the formation is inappropriate to be used against a particular opponent, then of course it should be classified as inferior. In fact, in some cases, illegal formations have been observed to be superior. But, some people have mentioned the possible purpose of having the 'illegal formation' system. This might prevent unexpected outcomes (referred here by many users as 'troll' results). But, still 'troll results' have been occurring. So, this game might be fooling you.