I am not fully aware (yet) how this will impact how I use Composr now (very early in my deployment) or into the future (which will be forum intense), but I hope you keep in mind I switched from Drupal as the forum functions there where not strong, where they appeared to be very strong here.
What impact will this have on forum function, and hence useability, I hope little?

Good.

I think this is an excellent idea as the current situation just wastes resources better spent elsewhere. As noted, there are converters for most the major forum systems and people with budgets will be able to pay to have any necessary changes made or to add a new importer for a foreign forum system. If there is much demand for forum drivers or converters not already available, this could be a great business opportunity for a third-party service (i.e., a paid service).

I think if keeping it hampers development then drop it however I think it is a feature that really separates Composr from all the competition and if possible it should remain no matter how little it is utilized. I started out with the thought of using the integration product and ended up just using Composrs own forum but it was the choice that attracted me.

I agree with Bob entirely on this.
Furthermore, the concept of a CMS is to manage content and there is nothing more massive than forum posts which requires perfect integration with the rest of the bits and pieces of information scattered on a vibrant website. The existing ability of Conversr to handle all this magnificently is rendered inefficient by using a third party forum. In fact it goes against the grain of what is Composr in the first place. I can appreciate situations such as the one Doc is referring to when there is little choice but to integrate a substantial existing forum with Composr as a first porting step, and in those cases I would advocate also using third party integration tools made by users interested by this functionality with some limited template guidance from Composr.