I've noticed that we've been able to find everything at retail (usually several times) - things are restocked consistently. Plus, there is a nice variety to the line. Sure, there are multiple versions of the Turtles, but she has at least a dozen villains/supporting characters as well in the first year+ of the line.
This echoes my experience with a different line I have been getting for my son, Disney's Planes. There was one character, Rochelle, that was packed at 1 per case for one wave. My SW experience warned me to expect a tough find on this one. I've seen this one on three or four different occasions, and it isn't because these things sit. Every store we frequent regularly turns over their stock (pegs empty then full then empty again).
I think other companies "get" it where Hasbro just doesn't.