1) They are so expensive
2) They don't offer any deterrent
3) They can offer false security
4) They don't imobilise the bike
They cost even more than a high end D-lock like a Fahg Mini. Theives will think the bike is not secured properly and will damage the bike trying to steal it. Even if they fail and then give up, they will probably kick the wheels in in frustration. These locking devices won't stop your saddle being stolen for example. Someone can just saw through the seatpost to take the saddle (it has happened). A D-lock on the front wheel will first need to be broken if the bike is to be ridden away, whereas locking skewers won't need to be broken first.
1) They are so expensive
2) They don't offer any deterrent
3) They can offer false security
4) They don't imobilise the bike
They cost even more than a high end D-lock like a Fahg Mini. Theives will think the bike is not secured properly and will damage the bike trying to steal it. Even if they fail and then give up, they will probably kick the wheels in in frustration. These locking devices won't stop your saddle being stolen for example. Someone can just saw through the seatpost to take the saddle (it has happened). A D-lock on the front wheel will first need to be broken if the bike is to be ridden away, whereas locking skewers won't need to be broken first.