Well, I would counter that you just made my point for me! The reason the app sucks is not that it doesn't connect to the bike - if you get someone to use it whilst actually on a bike they see the utility straight away (and indeed the base problem the app solves is how to get a decent exercise program going when your on-board computer is braindead, and hence can't be connected to). The problem is that it is very difficult to market. If I had asked the question 'Why does this suck?', people would have told me that it's because it doesn't connect to the bike, when in fact the real problem is elsewhere - asking the question would have got me the wrong answer, which was the point I was trying to make in my first point.
But I would counter your counter by saying that you identified late in the game that you were going to have a very hard time marketing the app due to people's expectations. Poor marketing and inability to get users to bite kills plenty of decent ideas. Not recognizing that early on is as damning as building something that collapses when you try to scale it.
I'm not saying your idea is a bad one. I'm just saying that getting criticism early on, even of a vague idea, can be very helpful. It can help you not only in implementing your idea, but also in implementing any marketing strategies or deciding how you want to position your product.
