As for why would I consider doing this to an 8b, I would try it to see what it sounds like. This exact mod is described in the consumer Instruction Manual that came with the amp in the 1960s when consumers owned soldering irons, so it's not really a sacrilege, just an option. I've read about numerous people doing it and reporting interesting sound. It can be reversed, and amps this old from this era have typically been worked on to stay at spec.Ultra linear, if done properly, causes the output section to behave as if it is triode rather than pentode or tetrode. Marantz did a pretty good job of getting the taps right on their output transformers (there was a patent in the area that prevented them from getting them perfect without infringement).
Depending on how the output section is set up (UL has more gain than triode operation) the amount of feedback that is present will change and may not be ideal. I think this is why Saul didn't offer the triode operation as a switch: it simply wasn't worth it given all the variables that arise! I have heard the model 9 in both UL and Triode; with that amp triode is the way to go. One thing to keep in mind: if you are operating the power tubes in triode mode, the correct plate to plate load you'll need will be lower. This **might** mean that you would use the screen taps for the plates of the power tubes. The correct cathode current will be different too, since there is no longer screen current with which to contend. Then there is the issue of whether you are driving the tube with the traditional control grid, or if you are using the screen grid as input, or both.
Put another way, but simply wiring the tubes in triode and otherwise making no other changes will be 100% inconclusive. You won't be able to tell if triode is better, you'll just be hearing how the amp works with that mod and not otherwise optimized.
If I were you, I'd build up an amp from scratch that allows you to work with these variables rather than risk damaging an output transformer, a power transformer or otherwise damaging the value of an amp such as this! If you want to mess with stuff like this, do it with a Dynaco ST-70, which uses the same power tube and is also UL operation. ST-70s are a lot easier to find, have much wider aftermarket support and are less expensive to own and repair. I can also guarantee that if you mess with the innards of a Marantz, unless you really have your chops down, the fact that you were in there will be patently obvious to anyone looking at the amp at a later date!