The most important thing when drafting in the NBA is to take the best player available. You can always sign free agents or make trades to balance positional need, but unless your a franchise like the Lakers who can always add talented stars who want to move to Hollywood via free agency or a forced trade, you need to be able to draft well. By the time your draftee matures what was once a positional strength can now be a deficiency and vice versa.
So while the Boston Celtics were well loaded with point guards this past season in Jrue Holiday, Derrick White, and Payton Pritchard, who knows what the future holds next season or even moreso three years from now.