Output modules are yet another set of collective victims of "Sexy Module Syndrome". Frankly, they're just not sexy, as a rule. But they're pretty essential, IMHO. It's convenient to have your level step-down inside your cab, for one thing. And if the module has ganged attenuation for stereo level control...well, bonus! But the critical thing with these is to find ones that have isolation and/or balancing transformers. For one thing, these help you avoid ground loop hum and "dirty" power noise. But also, transformer isolation is a sure-fire avoidance for passing DC through your audio output...because transformers don't pass DC, period. And as an extra benefit, you can overdrive and saturate output transformers to add a little warm crunchiness to your sound.
Like anything else that doesn't fit into Sexy Module Syndrome...they're not sexy, but that doesn't mean they're useless!