My personal experience is that towing my sand rail on its T-trailer, which was the same weight as my old 14 ft flatbed tandem.
The sand rail I didn't feel behind the truck and it didn't really affect the mileage at all. Even empty, I could feel the bigger trailer and it knocked a mpg off the truck.
I think its a combination of these things:
Less weight. With a LIGHTWEIGHT wheeling rig, this can be 15-20% reduction of weight, which isn't insignificant.
Lower profile. I think designed right, you could lower the towed vehicle 8" or so over a standard trailer and 12+" over a deckover. Wind resistance is a significant portion of what is sucking down your gas.
Less road drag: Single axle instead of two axles. No scrubbing turns, 1/2 the rolling resistance.
I bet you coudl gain 0.5-1mpg, and have a smaller feeling trailer with this setup.
BTW, to answer your original question, I think its OK to not have springs on the trailer, if you strap down your axles, not the body. At least with standard springs and shocks. I'd watch your shocks, though. Don't know if I'd do it with air shocks. I've had my Toyotas shocks leak and get hot while on the trailer, on a really bumpy road.,...