i always bid on the words that i get a positive roi for, whether i am in the top spots organically or not. If there are ads in the paid listings they will be getting some traffic from the term, so by you adding your listing to those as well you can increase the amount of traffic you get for that term.
one example -
we have a no 2 spot in the organic listings and are in position 3-4 in paid listings as well.
a competitor who has the no 1 spot in the paid listing gets more traffic on this term than we do with the no 2 organic and position 3-4 paid. meaning that in this industry, being in the top paid listings is more important than a reasonably good organic position.
This would not be the case in all industries. It is only on one of the sites we run we have seen this happen.
So it would be worth trying it and just keep an eye on how much you spend and on what your conversions are to make sure you are running at a positive roi.
Remember - for every visitor you do not get, a competitor will get. I would rather run on a lower roi and get the business rather than miss the sale. Keeping my market share up is important as long as it is profitable.
hope this helps you.