Well DT sealed them with insulation. The insulation they stuffed into the bottom of each pipe was more than enough to stop air flow. But it does not prevent water going down, bees and other bugs looking for a nice warm place to live.
They were never installed as a means to ventilate the attic area. To correctly do it would require an open area above the insulation and a cross vent at the rear of the trailer. It still does not mean the air will flow. The cross members with insulation will prevent a cross flow.
You have to remember the construction of the unit. Its consists of: Vinyl Ceiling, Sound deedening foam, Luan, R-19, 1/2 plywood, felt, Roof Vinyl. There is NO moisture barrier.
So the introduction of cool air at night against a warm ceiling can create dew. Simply put this roof was designed to NOT be vented. So long as it remains dry it won't ever need to be vented.
Here is a link to everything you wanted to know about flat roof venting and it also states that in many cases venting vs. non-venting had no significant difference when the roof started out dry. Only did if have an effect if the roof was wet.
http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd176_e.html