Its not as much of "making other cars fit" as it is "THS is a budget class and building 200r4's cost $3k a pop, whereas a th350 is half that and will last longer." If this is a budget-minded class, let in the people that are on budgets, and can't afford to spend $10k a year on 200r4's, but they can spend $2k on a 350 once a year.
After thinking it over, I'd not run in THS due to the trans rule. I just don't have faith in my 200r4 (no offense Lonnie, I'm sure your 200r4 in my car is stout, but I'm going to grad school and can't take the risk.).
I don't see how the trans being changed will give anyone an advantage.
Its not like going from an 8.5 to a 9" either. Going to a 9" requires more money, and costs more time to make work. 8.5" are cheaper. The 350/400 trans is cheaper, which is why people are asking for them.
Its not the performance gain, perse, its the "I'm a broke student getting his Masters degree and I cant afford to spend my loans on transmissions".