I think heavy duty trailers use channel for two reasons.
1. All surfaces can be treated and cleaned to minimize corrosion.
2. Channel WILL deflect without cracking and transferring stress to welds better than tube profiles. There are many instances in manufacturing where rigidity is not preferred over controlled flex. Trailer frames are a prime example.
I worked as a trailer engineer for some years. Dont know if anyone is interested, but here it comes:
-for light mid-axle trailers behind cars and vans, we used 2x2" tube spaceframes because the torsional stiffness gives a smoother ride, and at the same time a lower weight (given that the distance of the longitudinal tubes of the space frame is high enough)
-for the stuff from 7 ton upwards, we used I beams: these were all 5th wheels which are supported at the axle (or tandem gear) at the rear and the 5th wheel dish at the front, which prevents most vibration flexing on the road. But mid-axle tag trailers behind trucks, we also used I beam because the price of an open profile is lower than tube. Also, the torsional flexing problem gets smaller when the weight of the trailer increases.
For heavy equipment trailers, monotube section frames from 3/4" top plates and half inch vertical plates are the norm, also because of their torsional stiffness, as it helps them to better withstand the torsion of unequal loads of odd loads like machine parts and the such. Also, in this class of trailers, the technical reserves are much higher and often a special permit for oversize load is required anyways, so they are not as "engineered to the pound" as box trailers, which are generally constructed out of I beams.
For a lightweight trailer i'd want tubes, preferrably galvanised as it covers the inside too. This because the dirt cheap trailers built from brake pressed sheetmetal you'd find at the Dutch equivalent of Wal-mart, will flex even with just a strong crosswind: looks like you're towing a cardboard box behind...
Tandem and single axle, mid axle trailers, are supported in the mid and at the ball hitch: kinda like a triangle. this allows them to flex over the longitudinal axis.
In heavier trailers, the vibrational frequency of the chassis is much higher, so usually you wont get resonance from road vibrations.
That was a long winded story, but in general, tubes for mid axle trailers up to 7k, and I beams for anything over that weight was the rule of thumb where i worked. Thats my personal preference too, as my current 2.5 ton deck-over trailer, with a space frame from cold rolled C-channel, flexes quite more than the ones we used to build at my former employer.
oh, and one thing about them dumptrailers: open channels are easier to treat on all surfaces, but the open ends can also collect more dirt, which may stay on the flanges of the profiles... lots of pros and cons....
