There are three ways I can see myself responding in that situation:

• I did find the response helpful, I already know I found it helpful, I'd like to tell the responder I found it helpful, but (and obviously this is just me): the "please let me know" takes some of the fun out of it. I'm no longer telling them because I felt like it, I'm telling them because they asked, and I'm expecting them to wonder if I really meant it.
  
  It reminds me of trying to answer someone's question, "Do these pants make me look fat?"

• I didn't find the response especially helpful, I'm not sure how well the responder understood me... but for all I know I might not have understood them either, and they they were kind enough to answer, so there's no way I'd want to tell them, "No, I didn't find it helpful, "

• I'm still hanging out with their reply. It almost certainly touched a little bit on some issues of mine ("hit a nerve," as Flooded put it), or why would I have bothered to ask? I'm sure I don't fully understand the reply yet, nor know if it'll be helpful to me. For all I know, it could be a week or a year before I do. Am I going to come up with a way to tell the responder all that? If I do, will it be this year?

I can't speak for anyone else but when I don't say one way or the other if something was helpful to me, that most likely means I'm still hanging out with it -- and getting more out of it than if I'd said, "Yes, it was very helpful, thank you very much... [There, got that over with, now where was I? ]"