Hybrid change would compeatly shit on the concept of colour identity and quickly unravel the boundries of the format. If all it takes for me to use a card is being able to cast it, why do i give a shit about coloured ability costs?
Or worse, coloured mana production.
Why should white creature Kitchen Finks be mono green and Avacyn's Pilgrim be Selesnya?
Let's not even start with Phyrexian Mana pandora's box