Is it a bad idea to take the knocked in bat in the nets against bowling machine with those soft dimpled balls?
I think it's a pretty good way of knocking a bat in, although some may disagree due to the quality of the balls.
There are some differences between the balls I think, have only seen red or yellow myself and they've been fine. As long as your bat has had a decent amount of mallet time and the edges and toe are well prepared an hour or two on the machine will help both the knocking in process and gets you a bit of practice too - what's not to like!
Just like having a net, you still need to be sensible though, i'd start off slow and build the speed up gradually until you're comfortable with it.