Can somebody explain the appeal to me? Not trying to start a flame war, just trying to understand.
I dont think they are intended to compete with the other primes...rather to serve a complementary purposed to larger kits.
For example, I have an 11-16mm, a 17-70mm, and a 55-250...all lenses I like and that aren't "heavy" per say. I had a 200mm f/2.8 as well at one point. I'd take them backpacking, I'd take them in my boat kayaking, etc. On longer trips, the extra 4-5lbs of those lenses adds up quickly. It weighs me down and takes energy away, making me less excited to do photography. It affects the performance of my boat and takes up a lot of space.
Now, make that just a 24mm and 60+mm pancake (especially combined with an SL1 or TXi camera), and suddenly my total kit weights about <2lbs, instead of say 7-8lbs. I can take it in a smaller dry box, which also means less weight. There's also physical limitations to those boxes...I might need a pancake to even fit it in a box that works for me.
Mostly, just compare a 60D w 35mm f/2 IS vs an SL1 with 40mm f/2.8. It's a BIG difference in size: http://camerasize.com/compact/#100.368,448.345,ha,t
And the tradeoffs are relatively small. No swivel screen, lower fps, less weather sealing, etc. But, for people that need portability, those tradeoffs are often much better than going mirrorless if you still do sports shooting.