Temperature in the office environment needs to be regulated, if only to improve productivity. There are quite a few academic papers out there studying the effect of ambient temperature, within an office, on productivity. General conclusion is that we're most productive when the temperature is 22C.

HVAC also helps filter out external pollutants. External pollutants also negatively affect your productivity (see studies below).

The downside to an HVAC-equipped building is that should the system be poorly maintained, it can in fact spread diseases more quickly than a naturally ventilated one (sick building syndrome). This obviously negatively impacts your productivity.

Despite the possibility of SBS, I'm still of the opinion that the loss in productivity when people get sick is still less than the overall loss in productivity should the temperature not be controlled within the building (sickness affects a few people, temperature affects everyone). So my answer is yes, we do need aircon (and temperature control) in the office. It's by far the most consistent method of controlling the temperature around you.

Here are some of the studies. They specifically focus on an increase in temperature, but state that the reverse also applies.

Temperature effect on productivity:
http://eetd.lbl.gov/ie/pdf/LBNL-60946.pdf
http://eetd.lbl.gov/ie/pdf/LBNL-53191.pdf

Pollutant effect on productivity:
http://onlinelibrary.wiley.com/doi/1...003.x/abstract
http://onlinelibrary.wiley.com/doi/1...4.00218.x/full

There are many more studies I could quote, but I'm too lazy to go and dig them all up now.