Steel only has 12% of the conductivity of copper, so "all that steel" in the frame is not quite as significant. It works out to about the same resistance for 2 parallel 1/0 copper cables as one boxed fram rail. So it would basically be a wash as far as that goes.

In my installation, the more important point was maintaining a common connection point on the frame for all the heavy ground connections. If all the ground wires are connected to the same point, there is less chance of ground loops and other elusive electrical problems related to grounding. On the other hand, at $1.50 or more per foot of 1/0 cable, you could save some money by keeping the use of it to a minimum and connecting to the frame as close as possible.