I think these sorts of things are important, they help with logic and hand-eye co-ordination and encourage us to finish what we have started. I have found this is a problem for me sometimes, I find things especially physical things quite frustrating and can get angry and give up when I am capable of learning with patience. So if you specify jigsaw puzzles then yes it may seem unimportant but for me it teaches important skills. But these skills could potentially be taught some other way I guess.

My son and I are both bad writers and poor at JSPuzzles.

It take me ages to put together a puzzle and my writing is awful but doing activities like putting together jigsaws will help him with his fine motor skills. If he doesnt like jigsaws then maybe find something else he will enjoy but uses the same skills as putting together a jigsaw! It good that he plays the piano im sure that helps alot with his fine motor skils! :)

I tried to play the piano when I was younger but could only play with one or 2 fingers at a time, anything else I was too slow at.
As far as jigsaws go... I was doing one once and I got really frustrated because I couldn't do it, so my Stepfather threw it on the fire! He said I was annoying him. I never touched one again!

Working for many years with children with dyspraxia I have never come across children who have gone on in life to have a carreer in jigsaw building (but someone out there will prove me wrong) I deliver an exercise programme to overcome gross/fine motor skills and one of the things I discovered was. When children move as babies they train messages around the nueral pathways. In normal (sorry) these meassages go from 1-2-3-4-5 and this eventually becomes a rapid response over months to 1-2. Unfortuantely dyspraxic childrens messages go 1-5 and continue, so processing is slow. The same thing happens to their bodies in that when they want to move one part of their body it all goes. When I screen children I noticed that they all could not track an object and you need this to read and write. I realized that as their bodies had moved when they were babies they had also never learnt to fixate on a object and learnt to track. I do an exercise in which children throw a bean bag from left to right in an ark, they also have to watch it. They can also pick up beads everyday with the thumb and each finger in turn. A child did this and his tremor dissapeared. All 90 children can throw a bean bag,track and throw a ball which is one of the best for hand/eye co-ordination. But most of all there handwriting and reading improved.
I hope this helps but did you know that (normal)boys are the worst writers and are below average and unless he is going to be a writer I shouldn't worry.