So far not a single reason has been given towards favoring carrying over strength for a power based HB.
If you're going power, statistically, most of your BTs will come from power throughs, where we know through direct confirmation that strength plays a higher role than carrying. LTS follows the same rule. It is not possible to have enough SPs to invest into fakes, spin, pt, lts, etc all in the same player. Usually people focus on PT, a few other power tree skills and spin, leaving fakes out of the equation. Considering that, we have two skills where strength is more important than carrying and one where carrying is more important than strength. Now unless we're planning on using our power back in the elusive slider and on outside rushes a lot (which frankly means you're not talking about a power back but a combo back like my old one), spin will have a lower impact overall than power through.
All of this is direct statistics and facts confirmed by bort. Taking into consideration these as facts, again, WHY IS CARRYING MORE IMPORTANT THAN STRENGTH. Do be direct and factual, please. I understand if people said they had the same importance, but since it makes no sense to me that carrying would be bigger than strength IN THIS PARTICULAR TYPE OF PLAYER, I'm quite curious at what the rationale is behind it.