It is sort of an interesting discussion. Let me bring it back to the
Orion so it's a little more on topic.
The Orion allows you to disable either the mic level input or the line
level input, or run them both. (The unbalanced line level input
accessible through the aux connector and supplied breakout cable
is disabled by default). If modem tones are fed to the line level
input, the mic input can be completely disabled -- so you're not just
turning the gain down and taking the risk that something untoward
will get in that route.
Also, the DSP monitor function allows you to monitor the transmitted
signal whether it's sourced from mic or line, which should make it
reasonably straightforward to determine whether or not there is
garbage (hum, RF, hiss, or general chortle) on the signal in any
case. My experience so far is that if you don't hear it in the monitor,
it won't be on the transmitted signal -- and vice versa of course.
As I currently lack a TNC of any ilk, I've checked out using the line
level input with external audio gear and various microphones. Line
and mic input levels are settable separately, and the ability to switch
between them in the menus makes A/B comparison pretty easy.