Wireless can be easy to install but getting wireless performance is not. An easy installation is convenient yes, but does it work for every application? It can but doing it the right way could be difficult and/or expensive. Often times, the cheapest & most effective answer is a wired connection.
However, the trend in easy consumer access to inexpensive 802.11 devices has made it the “duct tape” of the networking world. This logic is dangerous. Why? Wireless is an open system, unlike wires (closed system) which means that the variables affecting performance in a specific location can only be estimated. 802.11 itself, in an effort to make wireless more accessible has made many trade-offs that make it often useless for anything but the least demanding applications.
Since we can only estimate conditions, there is a standards body (ITU-R) that provides models to help with accuracy. Model updates in the last decade have made drastic changes. The P.452 model update from revision 14 to 15, for instance, impacted the model calculations less favourably by 20dB. Decibel math is a way to do logarithm base 10 math (exponential) in your head using simple addition & subtraction. For example, a 3dB difference is double, whereas 10dB difference is 10 times. So 20dB means 10×10 worse or 100 times worse than the older revision. For the engineering types, wireless propagation modelling and the different models available are discussed in this whitepaper:
http://www.transfinite.com/papers/Propagation_Modelling.pdf