Is Danish Oil redundant to Polyurethane, or do both finishing agents provide complementary qualities?
Is Danish Oil redundant to Polyurethane?
Mostly yes, you'd normally use one or the other not both.
If you're already planning to use an oil-based varnish (any type) as the final finish on a project there's usually no reason to use "Danish oil" (or any similar oil/varnish blend) beforehand. Although this won't do any harm it adds an unnecessary step and yields no benefit.
There are two exceptions to this, the first is where the "Danish oil" is coloured so you're effectively using it as a stain.
The second would be for optical reasons, where you've done a comparative test and found it enhances the figure* or chatoyance of the wood more than the varnish applied directly to the bare wood. However, this enhancement is usually best done using straight oil, mainly because it's much cheaper (it can also give a more pronounced effect).
A hidden benefit of having oil on hand for doing this sort of thing is that you no longer need to buy a product like "Danish oil" because you can make your own version, very quickly and at a significant saving — all commercial versions of this type of finish greatly overcharge for one of the major components, the cheapest, the mineral spirits. This can sometimes be 75% or more of the contents of the tin!
*Often referred to as "popping the grain".
Danish Oil cures softer than polyurethane. So for me, it's worth it to do polyurethane over Danish Oil since my projects tend to be pieces that will be used (tables, boxes, cabinets).