It depends on how many people are in the channel and the expected purpose of the channel.
There is certainly some amount of information dissemination by having discussions in a public forum, but you have to weigh that against the information processing burden it adds to the people in the group.
Also, it may become searchable, but it's only findable if you and your communication partners use consistent, appropriate, and relatively unique keywords. There are likely better ways to document things; although, documentation is one of my worry areas, and I understand the desire to have something rather than nothing.
I still haven't figured this one out. Unless it is a criticism or "hey you broke this", my thought is the default should be a public channel, no?