Personally I see no problem relacing a wheel with the used spokes and nipples, but most of the time the decision is based on why you are doing the job. For instance, if you are replacing a rusty rim and the spokes are fine, reuse them. If on the other hand you are replacing a rim that has been in an accident it may well be a good idea to use new spokes. Other factors are labor/price/availability, as a mechanic working at a brand X shop I had to repair a dirt bike wheel, half the spokes were broken and many others had rust frozen nipples. Being that the spokes were only available as a complete set and the owner probably doesn't want to pay 75/hr to have me wrestle with the rusty nipples I grabbed a set of bolt cutters and chopped the old ones out and started fresh.
During a previous winter I took my wheels apart just to clean the pieces, I liked the result, but when I think about how much time I put into polishing those spokes (all 80 of them, one at a time), I now wish I'd just sprung for a set of stainless, oh well.
GB