I come from a judo background and have been in a "self defense situation" where I successfully employed a standing sankaku jime variation and gripped my own shirt the same way I would have a gi. I have also been choked unconscious with the t-shirt I was wearing in my younger years.
One of the biggest benefits of including strokes and strangulations in training is simply getting used to getting choked (mentioned earlier in this thread). Having been choked hundreds of times I can remain very calm as the "haze" sets in.
Perhaps I should say it is "less likely" rather than "unlikely" that a gi strangle would be used against you in a self-defence situation rather than a naked strangle -especially in view of the greater probability that it would more likely be an MMA guy rather than a high graded judo or BJJ guy that is likely to attack you. If only the naked strangles were included in aikido practice, an aikido person could get reasonably competent at defending against them and using them without sacrificing too much training time to strangles. Anyone who wanted to become competent at gi strangles as well could practice BJJ. This would also parallel what Tomiki Sensei did when he developed his style of aikido from standing judo techniques - throws that required grabbing of the gi were generally not incorporated in his system, whereas throws that required grabbing the naked arm or a soft atemi were included. His view was that if you wanted to practice the gi grabbing throws, then train in judo as well as his style of aikido.