Paying a fee does ensure priority, but it's also a thank-you for the miners, much in the same way it's common in some places to tip the person serving your food at a restaurant.  You can certainly send transactions without a fee, but it's not very polite.  Also, over time, the network will become more reliant on fees as the block reward for miners continues to halve.  If we don't provide adequate incentives for miners, they won't continue securing our transactions and that will cause problems.  This is something that sidechains, lightning network and other third party "solutions" fail to address.

Yeah they can be included for free and a lot of the time they are. However its all about incentive and priority. A miner can choose to ignore all TX's that are below a certian fee if they wish even if they only produce 1/10 full blocks.

People included fees with their transactions before the 1MB limit was enacted, they continued to include fees when we were nowhere near hitting the cap and they would still continue to include them if the cap was raised.