Some of us, speaking of myself personally, are not gifted martial artists, or even highly coordinated.
It took me quite a while to learn boxing footwork and how to connect my body and punches to that footwork. Now, I am learning Shotokan Karate. The footwork is different. The mechanics of how those connections are made are just enough different. Due to my non-giftedness in martial arts and that I originally learned footwork and body connection from another art, there are some simple Karate punches that drive me nuts... in the pat your head, rub your tummy, while standing on one foot and counting backwards, the odd numbers from 100, sort of way. Now, I am asked to do a block, that I am ok at, with a new footwork, then transition to a technique that I can barely do, by itself, while someone is punching at me, requiring me to blend with... all at the same time. First, I am going to get hit. (ask me how I know...) Second, I am not going to be able to get the combination right. Third, and most important, I will not be able to improve, no matter how many times I get hit. In fact, when I get tired of getting hit, I will start doing something else instead. Which means I will not ever be able to use this Karate approach in sparring, or any where else.
For me, it helps to work out the coordination for the combo first, so that I can do it reasonably well. Then ramp up the resistance and realism of the drills. You see some people that can punch the air properly, but put a target in front of them (focus mit) and they fall forward, and unbalance themselves trying to hit harder. You can see all kinds of people who train only hitting other people and heavy bags, that always fall into their punches, giving up their structure and balance. Having the other person hit back at them, is not going to help fix the issues with stance, balance and structure.
Maybe I am just the doofuss here, with serious coordination problems. But, learning things a bit at a time... breaking complex movements down to their simple components, certainly helps me to learn things quicker. So, for me, which drill is better: the one that gets me to do the technique properly or the one that has me do it against a realistic and resisting opponent? I need both (and a few in between) or I will never get it. (even then, I have my doubts...) They are different drills, that work on different parts. I need all the parts. Take one away, and I won't get it. But, maybe you have better students than I am... who knows.