No, sumwun, it means that the most probably, you'll end up catching the Pokemon before you reach those 30 turns anyway. Then again, there are some legendaries that have very low catch rate, but Dusk Balls work on most of them.
Another thing is Ultra Balls are found in the main Pokemart, so while buying repels, people stock up on Ultra Balls as well, instead of going over to the other side (terribly lazy, I know, but it is a general trend that is followed by most due to convenience).