Factorio Forums

The latest Friday Facts (number 306) says: "then they are left to deal with Freeplay style attacks."

However, after destroying all the biter bases there is still a small trickle of biters coming in from the West edge of the map.

Is this by design, or is it something left over from the previous versions?

Yes, this is intentional.
The bases on the map will try to "re-seed" eachother, and if there are no bases left, then it will spawn biters at the edge to do so.