The problem with this idea is that most people take a period of time to adjust to a different crank length. Some even initially have knee problems because of the different angles of the knee with different lengths. Also, when you change crank length you must change the seat height, all things else being equal. So I see no market for cranks that can change on the run so to speak. The Look cranks are a great idea for someone that is not sure of their size, for dealers that don't want to stock 3 sizes of cranks, for people that share a bike, for a father that wants to give a bike to a child later, etc. but changing on the fly provides no benefit that I can see.
Ahhh...here's some interesting thoughts related to that... Many years ago, I used to work at the Wheelsmith Inc. in Palo Alto, CA. While there, Miguel Indurains Tour de France winning time trial bike was put on display for a month at the store. Now I'm a taller rider, so I was happy to see that Big Mig did, indeed, use 180mm cranks on his TT bike. But a very interesting bit of information came to me through Ric Hjertberg (wheelsmith founder and now MadFiber wheel designer/founder) about Miguels fit... His personal mechanic said that even though Migule used both 175mm and 180mm cranks on his bikes, he NEVER changed his saddle position. At first, I thought this was ridiculous. But then, after some consideration, it started to make sense to me. So, I took my own bike and set it up just that way....so that two crank lengths could be used without altering the saddle position. I've been riding this way ever since and it is perfect. It is my opinion that we, as riders, do Not become familiar to a certain saddle position related to the lowest point on the pedaling circle, but rather that we become familiarized to a saddle height in relation to the bottom bracket center.
Here's how it works... You set your saddle as high as possible on the longest crank you are going to use, then whenever you put shorter cranks on, you will never be overextending...which is, or course, a most dangerous position to be in.
It actually aids the whole situation in that, when you have shorter cranks on, you naturally pedal faster, which necessitates a lower saddle in order to pedal smoothly.
I'm not saying it will work for everybody. But after trying it myself, I can't imagine doing it any other way.