Understood. I just worry a bit, because trying to decide which heresies are bad enough for exclusion, and which aren't, is a tricky business. There's certainly a slippery slope lying in wait down that road, although I think the Creed is probably durable enough to keep us from falling too far.
I do know a number of people who think that at least one large "cult" is still being allowed into the Christians-only forums, and who express a great deal of distress over this. I happen to think they're wrong, but I worry about the implications.
That said, as noted, I'm always going to be uncomfortable with any test of "true" Christianity other than asking people if, in their hearts, they consider themselves Christians. I would rather have fellowship with a few non-Christians who mistakenly believe themselves to be part of my faith than risk excluding even one Christian who has trouble with the words used for a given creed.
In the end, any policy can make two kinds of errors; it can mistakenly say "yes", or it can mistakenly say "no". Once, God's followers were very afraid of mistakenly saying "yes", and allowing someone impure or unclean to enter the temple. One day, a man came along who sought out the impure and unclean, and welcomed them into His kingdom; I believe this standard is the one we should follow.
The world is full of people who are full of the Holy Spirit, and essentially incompetent at theology. Doctrines such as the Trinity are beyond the comprehension of most, perhaps all, people. How shall we tell whether someone really understands the Trinity? Is it possible to accept it without understanding it? What does it mean to say that you "accept" a belief, if you don't understand it?
Any line drawn by mortals will have God's people both inside and outside that line.