That's great! Keep up the good work.
It isn't an all or nothing situation. If people's only choice is no PC or
used hardware and the time you have can't be better spent this makes a lot of
sense.
The problem is really when you start to look at doing things on a larger
scale. The resources to support the hardware become insurmountable. You need
to focus on a particular chipset/component in order to do it cost
effectively/efficiently. Labor is worth more generally (in much of the world
if not everywhere) than the money saved by refurbishing hardware. Refurbished
hardware is extremely expensive labor wise.
If your time is worthless (not to offend anyone) then used hardware is
probably always cheaper than new hardware. Generally speaking though on a
mass scale this isn't going to be true.
The best way to donate your time is not to used hardware unless this is the
only option. It's to new hardware. You can support much more variety of
hardware and get it to many more people by focusing your efforts on a single
component than trying to support a dozen components.