We don't need them anymore.

[Valence]

I think there's nothing to lose when you play ranked. You get the same as winning or losing in unranked but with the bonus of getting the chance to rank up if you win in ranked mode.

Yeah that's a good point, and I'd say once you reach above gold/platinum this is definitely something to keep in mind, you'll also be paired with players that don't keel over in comparison to your own level.
Casual games are all fun and good, but past a certain point they can make you aware very painfully of the skill discrepancy.