I was just about to ask the same question. My instinct is that it can be helpful to reference non-open sources as a means of describing/discussing the type of data/information being requested (or looked for).
It can also help with pseudo-reverse engineering methods of finding the same data (e.g. in this thread re. list of non-profits in the US the paid Guidestar use of 990's suggests an 'open' way of looking for similar records).
However, I think it really depends. In some cases (though by no means all), there are paid sources are really just well-organized aggregations of open data.
On the flip side, I don't think paid sources unto themselves would be helpful answers for questions on Open Data.