Is glutamine a useful supplement for athletes?

No. Taking glutamine orally does not seem to enhance athletic performance.
Studies say no. The clinical trials reveal little to no benefit.
You flunked the quiz. Glutamine isn't an essential AA. You make all the glutamine you need from glutamate, which is made from alanine or aspartic acid. If you consume exogenous glutamine, you just synthesize less. Excess glutamine, like all excess AAs, is burned as fuel & excreted as waste. So you'll have the wonderful experience of standing there with your fly open literally watching your money go down the toilet.