In my experience, I think you are correct... it's more on the lines of "We already have an extensive web testing project written in Selenium but are transitioning to TestComplete. How can we use what we already have without having to re-write it all for TestComplete?" That seems, to me, to be the best methodology. It could also be more on the lines if you don't have the Web module licensed since, I beleive, that Selenium integration is done via the Unit tests part of TestComplete which doesn't require the Web Module...
So... honestly, I think you're not going to gain anything new from adding Selenium to your existing web project... but, that's just my opinion.
