With purchasing spellbook you will be able to play ranked matches,join tournaments,rent cards etc. With the latest daily rewards update ronze league players gets less reward cards and dec and more credits and potions. I think it's still profitable to purchase spellbook for new players . You can rent cards and earn dec and reward cards without purchasing cards. If you rent the cards wisely then it's possible to make great returns.With just 2-3 usd worth of rented cards you can rise to silver league easily.