I was always told a tall deck BBC isa waste of time/money. Why is this, is it cause they didnt have a good bit of startin hp?

I don't think so. Mine just made 746hp two days ago on the dyno. Another plus to the tall deck is the longer rods put less stress on the side of the cylinder, this will not only help longevity but actually frees up some extra hp.
So whoever says stay away from them has no clue what they are talking about.