The length of OP has become somewhat of a meme that can make it daunting for new viewers to jump in. If
Toonami had started from episode 1, it would take five years to catch up even to where Funi's DVD release is, and that's assuming they stop releasing new episodes period. That says nothing of how far behind the Japanese airings that would put them as well. Running the show twice a week would help mitigate that.
Of course, running the show five times a week would help it even more. DBZ wouldn't have been as successful as it was if it aired once a week.
Not saying starting at episode 1 would have been worse or better, just that I can see the logic behind starting later.