Right now, we still get a pretty quick response even if we don't pay transaction fees. But if the network ever started getting clogged, you could pay a small transaction fee to be given higher priority. In the future, it is possible that transactions without fees take days, while transactions with fees go much quicker.
As the volume of transactions gets higher and the incentive for finding blocks gets lower (bitcoins per block get divided by 2 every 4 years, and difficulty is rising quickly), isn't there a risk that in the future, fees become the main incentive, increase dramatically in price (and also become unavoidable)? Then doing a bitcoin transaction would be for rich people only. We certainly don't want Bitcoin to become a rich people tool those who can afford the high taxes.