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Tosh.
Training involves training the gut to digest what you will be using in races as well as the rest of it (legs, CV, core, whatever). If you're tapping out 200W while watching a Simpsons episode then it's probably not necessary but if you're doing an 8hr tempo ride on a turbo then fueling is important and fueling like you'd be racing is also important.
hippy-
I think 8hr tempo rides may possibly make you an exceptional case. Personally speaking the longest race I'll do is 120km, you don't need to train your gut for that. If I did 2hrs + on a turbo I'd be minded to eat normal food (dates, nuts etc). You could even put a lovely little spread of food on a plate next to you because you know, you're inside on a turbo, rather than struggling to get something down mid-race hoping that someone doesn't attack before you've finished chewing/swallowing.
Currid
Eating gels on training rides is bad enough, eating gels on the turbo is completely mental.