You've seen
565bbchevy and
oldsofb factory filters in pictures above. And they've been in there for 30+ years. They didn't look moldy to me. Not to knock your A/C guy, but GM knew there was a drain hole, too. They put it there. But there had to be a reason they felt they needed this filter...unless it was the Chief Engineer's idea, so nobody would be likely to argue against it. The picture evidence is clear to me that mold doesn't seem to be a huge issue using the filter. I'm not sure I like the idea of it disintegrating, though, but I guess nobody considered that going in. Those bits and pieces could end up on the heater core or in the cabin. So there's that to consider.
Not sure when the filter, if used, got in there on some of the units, but I would suspect it was from Harrison. I don't think it was at the sub-assembly level when they assembled the HVAC units to be shipped to the factories, because normally they would just take components out of the supply box and put the completed sub-assemblies in place. Maybe they put the foam seals on them and slapped them in. Not positive, though. Just musing.
As far as the evaporator staying wet, the fan should be moving some air across the evaporator most of the time anyway even when the compressor is off, unless the head unit switch is in OFF, as well as the filter being open cell foam which probably drains water fairly well anyway, so maybe that was a consideration? If your HVAC system is smelly or you suspect mold/mildew, you can always get some products made for evaporator cleaning for mold/mildew removal. Just take out the blower resistor or if you need a bit more room, take out the fan to at least access the upstream side of the evaporator.
This is how I would approach it. If you don't have it now, you likely won't have the clips and pins for it anyway. So why worry? If you've been running it without one with no issues, again, likely no worries. If you decide to put it in, it can't be any worse than the factory install since it would be new. An extraneous part that isn't used will not cause any issues. Your call. Either way you decide, always keep your flapper clean.
🙂