so when i go to "New Tone" and start a set up from scratch, the amplifier is automatically set with a particular cabinet and microphone.
for example, the default for the J-45 BRT is the 412 Breenback 25 and the 421 Dynamic. and the J-800's default is the 412 Brit T-75, and a 57 Off Axis
question: is there any practical reason to NOT switch cabinet and mic simulations, in order to get different tones?
I don't think there's any particular reason NOT to switch.
That's usually the 1st thing I do when building a new tone.
For example: If using the JCM800 to build a new tone/patch for classic rock tone, first I will change the cabinet to Black30's.
Then either change the mic to the 57off-axis or 87condo to get as close to the tone i'm going for.
That way your not having to rely on more affects/eq's/pedals to achieve the same sound.
You would be surprised at how much a cabinet & mic can change the dynamic of a tone.
Hope this helps!