Extra drag and friction turning another Diff and lower ground clearance on the trailer. So you are on the trail your brake an axle so now you have to tear down your trailer to get your rig rolling so now you leave your trailer on the trail until you can get more parts. Toss the spare parts into your trailer makes more sense to me and most frames are not set up well for just using the bed as a trailer.
Points noted!
I wasn't thinking so much of being able to rob the parts from the trailer during a break down, as I was thinking of carring one spare that fit both rigs?
I see what you mean regarding drag from the diff, and low clearance.
Thanks for the input, that's why I love these groups.