I don't know if I would be keen on this either personally, because as soon as you open up one forum (even read only)...there will always be other people finding reasons as to why we should do this for other sections of the forums.
There may also be customers who don't mind sharing their URL to the community in the showcase forums, but don't necessarily want to share outside of that..
I think at the end of the day, if someone visits the showcase on the homepage, this should give them an idea of what you can do with ClassiPress in a much easier way than trawling through the forums and looking at individual threads to get URLs. If it's enough for them to go ahead and buy it, then they'll be able to access the forums and seek support from the community or from individuals with regards to customizing their theme. That's just my opinion anyway ;)