I think Pat makes many great points.
Overall, I don't think if you post a song here, you owe anyone anything. IF they choose to step up and help in anyway way they can, great! That's up to them. No one makes you do it; it's simply a choice.
For me, another choice, is in posting songs...or not to. You have to know your audience. The style(s) of music I write, tend to be more to the "pop" side of things. Not necessarily what the forum considers pop, but what current radio considers pop...or pop country.
I get what I like to write isn't everyone's thing here..and isn't by a long shot from what I've seen. As a result, I don't see much value in posting things I feel people don't want to hear. It benefits almost no one.
So, I have posted 1 partial song on the forums. I appreciated how nice everyone was with it too! I was just figuring out the program at the time.
The flip side is, I do listen to a lot of the music on here. I don't judge it by the standards of what I do or don't do. They only person I really compare that person to is the person they were last week, month, or year. As a general rule, I see growth in peoples skills. That's exciting to me!
I very seldom comment, because, I'm not asked to. There is the general asking for comments, but everyone does a great job already. There was a time I did, but explanations that end up in further explanations and, at times, defending an opinion seemed pointless.
Now, I choose to just enjoy. And there is MUCH to enjoy!
Overall, my take is post your songs if you would like to. Comment if you would like to. It's really not more complicated than that to me.