The point of practicing against random DI is that you learn to cover every option it is possible for you to cover on reaction. No, your opponent's DI is not random, that is not the point. The point is to practice the best punish possible. Practicing against random DI forces you to react to how your opponent DIs, not read their DI. As long as the reaction is not super frame tight, reacting is always better than reading. Comboing in this way means you aren't unnecessarily reliant on reads; in other words, it doesn't matter who you are comboing, mango or a cpu. You have everything covered. There is nothing wrong with reads either but reads when you could have just practiced reacting are bad. So if you want to practice the bare minimum tech skill of comboing no DI, then yeah, no DI is good. But if you want to practice optimally comboing your opponent, there isn't any justification for the CPU having no DI. And for most people, there really isn't much point practicing against CPUs beyond trying to optimally combo.