To cut a long story short, whenever you repeat a technique or sequence of techniques, you are programming your body to not only remember, but to execute that exact same pattern whenever you're in that same environment.
So when you're drilling with a training partner there has to be a give and take. To elaborate, one partner is executing the move and one is allowing the move to be executed on himself. This has many detrimental elements to it. Not only are you training bad habits, but you're programming your body to become complacent, especially your mind.
An argument can be held about whether you're programming the desired techniques more than the bad habits, or vice versa.
I personally believe that it would be more like 60%/40%. The one practicing the technique being the 60% and the one practicing the movement to allow his partner to hit his attack being the 40%, but it would be impossible to test. However it does raise a good thought! How much in your training are you developing bad habits through the process of drilling?
There are many types of people learning jiu jitsu. And it's super important that you figure out which ways of learning suit you better, and find an academy that actors for it! Whether online or in person.
There's no doubt in my mind, if you're the creative type, then you need to be learning concepts. And if you're the procedural type, then drilling is going to suit you better!
If you are a creative type and you feel the drilling makes you stiff and rigid or bores you, then I invite you to give me a chance to show you a conceptual approach that I stand behind. In the Art Of Learning Jiu Jitsu I explain how to train, think and ultimately create your own style of jiu jitsu.