The issue with >40% is it's getting to technical levels. Need to be concerned with CNS, oxygen clock, MOD's, etc. Or to put it another way, richer than 40% is pointless for a single gas dive.They probably want people to be a little familiar with nitrox before taking on a more detailed course. I honestly don't quite understand what the point of TDI nitrox is because it doesn't give you anything that computer nitrox doesn't and if you want to use higher than 40%, you'll need to take advanced nitrox anyway.
It comes into its own when diving with a bottom gas and deco gas along with the correct decompression procedures.