A frequent disguised outing involves some of their friends on the staff. Celestia and Luna put on their disguises and some uniforms and head out for some time that doesn’t involve turning heads. It has the side benefit of helping the ponies they’re with relax a bit more and accept the pony under the crown.
 
As for how necessary, I subscribe to the idea that they aren’t true immortals. They can be killed, though they are hardier than a regular pony. And to that end, the more eyes watching for danger, the better. A princess can take down a more powerful foe than a guard, but the princess needs the luxury to focus on things other than just watching her back.