It depends on what you put on that network. If you just want to provide a free Internet access then you can probably get away with it. Janitors, delivery people, and night vigils will benefit from the service.
If the wireless network is for anything else related to work, then you have a big problem. Normal WiFi has a nominal range of about 100 meters outdoors, but it is just an average. As the Wikipedia page says:
Outdoor ranges can be improved to many kilometers through the use of high gain directional antennas at the router and remote device(s).
Meaning that someone who is less than one kilometer away from your access point (so this could be someone on the ground, outside of the skyscraper) may be able to talk to your access point, if he uses some specific hardware and does not mind going beyond local power regulations for radio emissions. If that's just for some Internet access, probabilities are low that someone would actually do it (except possibly as a way to cover his tracks), but if there is anything of value on your network, then this will happen.