Our dream minds are disconnected from our muscles, correct, but you actually can create muscle memory in the dream world. The aforementioned limits still apply, but if you practice snowboarding all winter, then practice in your LDs all summer, it will help build muscle memory. Same with most other sports, there's just the previous experience requisite, and with some more complex sports, the muscle memory you build in your dreams won't accurately be reflected on your waking self. There's a limit to how much the mind can accurately reproduce, and sometimes when you're lucid, you will subconsciously shift the dream so that you do well (Like playing basketball in a dream could cause you to make shots that normally you would have missed).

In general, LDs can be used to practice things that are difficult to practice regularly in real life, but it's always much better to just practice in real life.