WiFi battery powered contact sensors are not very common, due to the power-hungry nature of WiFi. Perhaps improvements in WiFi technology will improve this over time.
IMHO, if one requires a bullet-proof home security system, then one should buy a dedicated, battery backed up, redundant cellular network, professionally monitored home security system.
If, on the other hand, one has NO current home security system, and would like a low-cost, mostly-reliable, self-monitored home security system, then using a Home Automation hub is an option. Instead of 99.999% up-time like a dedicated, professionally installed and monitored system, it will probably provide ~95%+ up-time.
For many, going from NO Alarm System, to something that is 95%+ reliable is better than nothing, and much more affordable.
Personally, if I lived in a location where I felt an alarm system would be truly beneficial, I would opt either for a professional system OR a very large, grumpy dog. If I lived in a low crime area, I would think adding a little bit of peace of mind from a home automation hub would be better than nothing.
Samsung attempted to industrialize their home automation hub for alarm system duties by partnering with ADT. This required special ADT Contact Sensors, for the same reasons being highlighted by other in this thread. However, we see how well the ST/ADT marriage has gone...with Samsung dropping the product line completely, leaving users of the ADT ST Hub less than thrilled with their purchase. Many Lowes Iris users bought the ADT ST Hub when Iris was shut down, as they relied on Iris as a home security solution. I feel bad for them, as I am sure they have now been very soured by the home automation hub experience...