Is there any reason not to farm Terramorphous on Normal mode?
There is absolutely no reason to, except for the challenge, farm Terramorphous on TVHM. All those getting better loot and all those theories are lies.
Terramorphous becomes easier on normal mode without the health and damage multipliers and it is recommended to farm him on normal mode.
The only reason to farm Terramorphous on TVHM is if you are looking for a real challenge.