I'm sure the weld helped Toyota save a step during assembly. Maybe allows one guy to throw the rear together and eliminates a job. That'd be big bucks in a production line. Not sure why the mechanic had to break it loose... all he did was lower the axle to add spacers. More mystery.
Yes, I do know that all the springs are marked though I don't know why Toyota has been doing such for at least 20+ years (at least with my observations in the 80 series. Examples from my FZJ80:
Ummm, actually, that tack weld, the process to perform it and the equipment necessary on the assembly line is a major cost/efficiency negative. I don't believe that is an OEM job. I've only worked on FJC's older than yours, maybe 5 different suspension upgrades, never encountered a spring welded to the cup. Rear spring removal has always been an easy and simple process, pulling them out by hand (in fact, most fall out using the best technique) with little effort, let alone breaking off a tack weld to do so.
In fact, the nature of the assembly requires some movement between the spring and cup during compression and rebound. Fixing the spring in place will cause binding, creaking and compromise the material by introducing heat. Additionally, paint would need to be applied to the weld sight, adding more time to perform and wait for the paint to dry, highly compromising the renowned efficiency of a Toyota assembly line. If this were the case, one would expect this across platforms in Toyota assembly lines, not exclusively the FJC. If Toyota is so smart to save time and efficiency during the assembly process, one would expect other manufacturers to adopt similar processes. Yet...
Further, installing a new spring on top of that tack weld will create a "stress riser" for the new spring, creating a point where the new spring will be compromised. For the installer to put a spring on top of that weld is creating a future problem for you. To have not ground that weld down, smoothed the surface and resprayed will ruin the spring, invite rust and create a nice squeak, creak, annoying noise for your future enjoyment.
Lastly, one would see thread after thread, aftermarket instructions and much whining and complaining across boards regarding this issue. Every manufacturer would give instructions how to remedy this production anomaly, how to fix and avoid their own future issues by having a tack weld in the spring cup.
If I were in your shoes, I'd find another mechanic/installer, have those tack welds removed, the area smoothed and painted, check the spring to see if it's worn and repaint it also. I still have no opinion regarding the difference in front springs, don't find it impossible but highly questionable, yet, without any firm evidence, I have to do more research.
