Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
If you run a nonfree program on your computer, it denies your freedom; the main one harmed is you. Your usage of it can harm others indirectly, by encouraging development of that nonfree program. If you make a promise not to redistribute the program to others, you do wrong, because breaking such a promise is bad and keeping it is worse. Still, the main direct harm is to you.
It is even worse if you recommend that others run the nonfree program, or lead them to do so. When you do that, you're leading them to give up their freedom. Thus, what we should avoid most firmly is leading or encouraging others to run nonfree software. (Where the program uses a secret protocol for communication, as in the case of Skype, your own use of it pressures others to use it too, so it is especially important to reject any use of these programs.)
But there is one special case where using some nonfree software, and even urging others to use it, can be a positive thing. That's when the use of the nonfree software aims directly at putting an end to the use of that very same nonfree software.