Deck maintenance is critical, they generally are a large investment and without protecting them you will be up for expensive replacement costs. I have carried out many deck maintenance jobs in the last few years with the results leaving it looking better and protected for another year. Water based or oil coatings, I have seen great results with both and it is personal choice as to which way you go. Living in a harsh climate with extremes of hot and cold, it is imperative that maintenance is done on a regular basis, a minimum of yearly in the hills.
Build up of dirt, moss, mould and lichens all speed up timber degradation, regular cleaning will even help reduce damage.
An example of degradation in treated pine decking in the Adelaide Hills. Regular oiling would have slowed the end grain rot. This area of the deck is unstable and needs replacing.