I find this forum can help give perspective. Like, I might think something my T is doing that bothers me is totally normal, but on here, can find out that maybe it's not so common. And it can really help to find other people who have been through similar things with their therapists--this site really helped me when I was first feeling transference for MC (a couple years ago...) and made me feel less alone. I can see why therapists might not like it though. In a way, it can be like a bunch of employees going to lunch together and *****ing about their boss/job. You're right that it can be important to take issues you're having to your T, rather than just talking about them on here. But here can help give perspective and validation. So it doesn't just feel like the client vs. the authority figure...

Edited to add: Plus, if your real-life friends/family members aren't in therapy (or haven't had similar experiences), it can be weird to talk to them about it. Like my mom being all, "Shouldn't you be done with therapy by now?" And some friends have been generally supportive, but also seemed a bit bewildered when I talked about transference. Like, "You should just stop seeing this person!" Well...it's not that easy...