During really hot times of the year when your interior is scorching from sitting in the parking lot, your A/C will struggle to cool it down when it's set permanently on recirculate. Outside air might be hot but inside is WAY hotter.
Also, any stinky smells will be harder to get out of the truck. I've noticed this with the F-word work truck I drive which has a habit of always selecting recirculate after the key has been off. Apparently it's "smart" and does this when it detects a high outside temperature.
If you're on a long trip then you want outside air coming in, especially if you've got multiple people in the truck - y'all have been spewing Co2 into the cab the whole time. Mind you with these old trucks at this point they're far from airtight so it may not be that big of an issue.
Richard