Pencil is pretty good. In general I feel like people trust something they pay for most of the time(unfortunately)
Balsamiq is nice for making something that you want to show to clients I.e. email or put up on a monitor. It's extremely inexpensive for what it does.
As a counter your point, I think a lot of professionals prefer something that is more likely be continually developed and supported in the future - a lot of free things go away
or stop getting developed after a few years.