I have never taken notice of anything anyone has said about walkers, except for the fact about them being dangerous around stairs!!

I have had all of mine in walkers until the have managed to try and climb out and thats when I put it away.None of my children have any problem walking, and in fact it usually made them walk earlier then normal.As long as the child has their feet flat on the ground then it's ok.
As for MCHN I tend not to tell them much as they think they now more about your family then you do!
Not to tell ppl they are wrong about their feelings towards them, but if Jack loves using it then let him use it!