As someone still trying to make up my mind who to vote for in the leadership race, I don't yet feel like I completely understand why she is supporting Brian Topp.
We ask tough questions. Libby answers them, and I think she did great.
But we have to realize even more so when it comes to the candidates themselves- but your concern is an example of one for which it was difficult for Libby to say everything- they are only going to go so far in answering tough questions. It is up to us to make what we can and what we will of what the candidates do say.
ETA: One thing that is tough to answer is questions that to answer with complete forthrightness requires bumping up against the strategy choices of your campaign. In the end, if not immediately, you can NEVER look anything but a crass opportunist doing that. Its a hopeless minfield. Small wonder politicaly experienced people do not venture there.