I walked three Caminos this year and my findings are the following:
1) Portuguese Camino - used a wooden staff for half the walk and found it useful for the muddy, steep and uneven bits. Distance wise not really necessary.
2) French Camino - started with nothing, bought a wooden staff fairly soon and in Leon bought hiking poles as I was getting pain in right knee and ankle due to uneven taxing of that side due to staff in left hand. After that poles helped to support me evenly when needed and were light to carry.
3) Via de la Plata/Camino Sanabres - Bought some top quality , light poles from Leki before leaving and when needed they were brilliant and meant I had no trouble with ankles or knees the whole 1000kms.
My advice, dependant on distance, age, fitness, joints and muscles, is use poles. Especially for long distance walking they keep you in balance more than a staff and spread and lighten the load and taxing of your body on the difficult parts of the trail. Most of the time I didnt use them on the trails but when the going got tough or I felt I needed that extra support they made a vital difference in keeping me on the move and preventing injury or tendonitis.
In a nutshell the pros and cons:
Nothing:
Pros: in balance, hastlefree, no extra weight or something to leave behind
cons: no support on muddy, rocky, wet or steep paths (both for descent and ascent)
Wooden Staff:
Pros: You feel like a real pilgrim and look like one!
, extra support, becomes a companion (emotional attachment occurs), good to scare away dogs or other human or animal pests. You can engrave it during stops. Its wood so feels natural in hand and bonding occurs...
Cons: On longer and harder routes leads to an unbalanced walk and extra taxing of other leg, heavy, you tend to forget it and need more than one on the trip.....
Poles:
Pros: Light, telescopic, you walk in balance, helps reduce taxing on knees and ankles by up to 10-20%, useful in mud, stony and watery bits, helps braking going down mountains and climbing going up.
Cons: 2 of them, unhandy on planes although I just put them in my rucksack and in the hold which worked fine, not handy for fighting off aggressive dogs due to flimsy design and weight (doesnt inspire confidence at that moment!), you look like a nordic walker!
Well thats about all folks!