The cheaper Bitcoins are the more people can afford to access some significant number of them. Long periods of cheap Bitcoin prices mean more even and widespread distribution.
I don't think a "significant number" matters, 1 dollar in BTC will always be 1 dollar in BTC, no matter if it represented bu the figure of 0,0000003 BTC or 12BTC.
Ultimately it matters how many BTC you, as an individual, have relative to everyone else because it determines your purchasing power.
In the early days people were mining and buying
thousands of bitcoins very easily. Today that's far out of most people's reach. Yet we're
still in the very early adopter phase of this. Bitcoin isn't even close to being considered mainstream. By the time it is buying even a single bitcoin will most likely be out of most people's reach.
Now imagine the very last people to own BTCs, the people now considered poor, people in Rwanda or Argentina for example. If the most they can ever afford is 1 Satoshi (.00000001) then their purchasing power will never amount to significant participation in the Bitcoin economy.
The longer prices are low the better chance for better distribution you'll get. Unfortunately this still won't be enough for the majority of the world's poor, by the time they jump in.