An excellent question and I also think it is not good for the customer or DF. If DF thinks they are the way to sell certain sets, like the Mega packs, then I can see the point. Yes, it is nice to get a special piece for getting a lot of one sub-theme..."Is having chase pieces a customer friendly practice?"
as long as they are not monster sewers giant rats
...but when the numbers get too high is is impossible for most (90%+ of DF customers) to get all of them. That can be frustrating.
DF did try to make getting oddball pieces easier with the Troves in KSV, and I appreciate that.
As far as the effects of KSV chase pieces, I cannot see how anyone, even DF can know how they work as KSV has not hit retail yet. It will be interesting to see how many people get a set for the Chase, even if they do not really want a lot o the other pieces. I really want that two-headed crocodile and I may end up getting the encounter for it. Every time I see the Imperial streets and that Lion I get tempted, though I do not need all those streets. For sure!!!
It will be interesting to see.