I'm no expert! But we all have opinions.
For explosive/flammable/contamination critical installations, definitely pressurize the enclosure so leaks go out and you aren't pulling in bad stuff. This requires you actually design the enclosure. Knowingly consider CFM and size fans and all exhaust port sizes. Otherwise you will not achieve a higher pressure. Think about it, if you just remove one wall, you can pull air in with a fan but the enclosure will be ambient pressure, no positive delta P. Exit velocity has to be high enough to generate a loss.
For aerator cabinets, I doubt we have critical risks so we DIY the design and build and get by with it.
I get the value of filtering BOTH inlet and outlet. A wasp can go in either side when fan is off. You can filter upstream of a fan on the inlet side. We filter inlet air regularly in our line of work... up to 50 lb/sec in one of my favorite playgrounds at work. Not a Lowe's item though.
