I agree that for the whales tier, this is not a negligible factor, particularly since many are powering down. In fact I did investigate which whales are powering down, and determined the effect is roughly ~0.05% per day or ~0.4% per week for the tier.
The point though is that this is about overall Steem Power distribution - and powering down is just another way the distribution changes.
Like you point out in the other reply, it would certainly be interesting to see what the exact breakdown is - how much of it is due to powering up/down, author rewards and curation rewards? This is not something I have the data for, and will probably need an automated tool of some kind, which is way above my pay grade to create. :) All I can do right now is see how the overall distribution is changing. By "irrelevant", I didn't mean irrelevant in general, but just irrelevant specifically to this being about overall distribution trends.
Also, the changes are pretty drastic in some cases. For example, the minnows have gone up by 35% in the last 2 weeks. Sure, they are still only 2%, but a rise from 1.4% to 1.9% is pretty significant within their own niche.
Thank you for your inputs!