Avro Arrow wrote:
you'd probably still want to decontaminate to get rid of any contaminant that had attached itself to the exterior of the suit
Bingo. I interpreted the cheap costumes as a budgetary constraint of the show. If you want to be literal about it, then the wire mesh helmets of the spacesuits seen in "The Tholian Web" would not stand up well in a vacuum, even if they do keep the mosquitoes out.
The transporter chamber decon would only effect the suits anyway. If Joe had whipped off his laundry and taken decontamination naked, it would not have eliminated the bug.
(I believe some of the novels made decontamination—even of infections—a function of the transporter's buffer circuits. Being able to "filter out" undesirable patterns would turn the transporter into a real miracle machine. No Brundle-Flies in STAR TREK.)