I can't really answer this in terms of relationships or of al-anon but with respect to forums I think they are there as a cyber community to be used where people find it helpful.
Just like any community there can be disputes and each of us learns as we go along about the culture and expectations within a group. Also a group can bring back memories we feel we should leave behind and if that's the case I think leaving a group can be a good thing. As long as any leaving is done politely, with respect and tolerance - then return is always possible.