I would say there's no advantage in replacing springs that are still doing their job, and appear to be able to do so for the foreseeable future. So, in reasonable condition (a little surface rust isn't anything much to worry about, and can be dealt with) and still nice and snappy.
Piecemeal replacement of springs can lead to the hole in the pillar becoming enlarged...and thus increasing the size of spring required - which will have a detrimental effect on the action.
It's also the case that old springs have proven their worth...whereas a new spring might be quite brittle. The quality of springs these days is not what it used to be.
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