HOLLYWOOD, June 21, 2000 - Ever seen Final Destination or Frequency? If you haven't, chances are someone within your rock-throwing range has. See, people -- a lot of them, we might add -- have been buying tickets to these films week after week after week, making them the year's modest, if not surprise, hits. Which bring us to this perplexing question: In a box-office world dominated by "M-I:2" blockbusters, how do you account for throwaway flicks (see opening paragraph) that somehow manage to hang around long enough to rake in the dough, while higher-profile flicks like Jackie Chan's "Shanghai Noon" fizzle (relatively speaking) despite high expectations. Just to get an idea of what we mean, consider this: It was just last weekend that the two-month-old "Frequency," which bowed in the No. 3 spot on its opening weekend (not a super- strong showing), finally fell out of the