I think monitoring the forums would be a good way to gauge customer impact of features and new product roll outs, asses the way your customers are responding to the product and possibly your support, and if possible to assist in getting your customers working.
if your customers can not get your solution to work, you will loose them and then get no money at all.
Paying for support is the premium way of getting fast support with an individual who is accountable and responsible for their responses - usually they give you the answer as well since you are paying for that. Paid support also fixes the problems for you themselves in a lot of cases so long as you are willing to provide access for them.
People on forums are "just trying to help" and may be delayed in their responses, not be prefectly accurate, or provide info to help guide you while not necessarily giving you the answer, and allowing you to do the work.
There are several differences in free and paid for support, but having both types will increase the confidence the community has in the vendor, knowing that if something breaks they can get help is a good assurance when buying the product.
if you feel that you are on your own most people wont bother with the product as a whole (in general).
just my two cents.