IME it is worth it to have a very well defined contact arrangement and then throw some ADDITIONAL flexibility on top of it.
In the time before we had the defined contact order for DS, his dad was so informal about contact that it was not that I had to re arrange my plans to work around his "need" for flexibility all the time, I ended up loosing my job and spending more than a year in JSA thanks to him.
The effects on DS were also very bad, he was constantly hurt by his father not showing up or leaving him under the care of other people in contact days. DS was also very stressed out as every time his dad got angry, he would tell us he was changing contact arrangements. It was a living nightmare, stressful for DS because he preferred to be in one house for most of the time, and stressful for me because I knew he was not going to spend that extra time with his dad (dad travels a lot).
At the end, I took him to court for a defined contact order. You can't imagine the relief I felt at knowing that finally he couldn't be messing up with us anymore. That the contact arrangements we had was going to be respected.... It didn't last long, as he didn't get his way, he decided not to see him anymore until he is older. Again more uncertainity than DS needs. I really have no idea when he is going to pop up, he hasn't in 2 months, but with such chaotic behaviour, I don't know when he is suddenly going to come back and what would be the effects for DS.
The only thing that gives me a bit of peace is knowing that it would be very difficult for him to get to have more contact with DS than what is already agreed in contact order, contact that he decided not to have anyway.