Seeing as I train in both BJJ and submission grappling I find myself in the middle of this argument. I agree with the principle of sport-specific training, especially if your weekly training time is limited. However I also think that training with a gi has made my no-gi wrestling more technical, and that submission grappling (without the gi) has made my gi game more explosive.
One very important factor to consider in this debate is the issue of self-defense. I think that it is vital to expose yourself to different training environments if you are serious about self defense. That means occasionally venturing outside of your comfort zone and training in different environments.
Even the most dedicated BJJ competitor should occasionally roll around without his gi, and do some light vale tudo sessions. The goal here is not to turn him into an ultimate fighter, just to familiarize him with the fast and slippery world of gi-less grappling. This will help you learn how to apply your BJJ skills in a rough-and-tumble self-defense situation.
Similarly I think that submission grapplers and people training for mixed martial arts should occasionally put the gi on and enter into the world of lapel chokes and gi entanglements. You may be surprised at how the gi changes your game by adding friction and gripping options. As someone once said: “don’t fear the gi – in real life self defense your opponent may not be naked”. Spending at least 5 to 10% of your training time outside your comfort zone (i.e. on the other side of the gi vs. no-gi debate) is important for developing self-defense skills and becoming a well-rounded martial grappler.
One thing is for sure – the gi vs. no-gi debate will go on. Just make sure you’ve experienced both sides of the debate before making your decisions.