Well there are reasons to run a 4* card over a 5* star depending on what you wanna achieve with the deck. For example, there are no 5* Scion (that I know of) so you have to use a 4* in an Ascension deck, or if the the numbers on one go very well with a Plus/Same deck.

There are uses, just not as varies as you'd think.

Personally I think SE should scrap the current system and put in a * system, where you can use any combination of cards as you like as long as it doesn't go over the limit.