Office blocks are typically poorly defensible. Doorways are made of cheap, weak materials like glass, and existing security is usually designed for summoning police or scaring off burglars.Offices typically have little in the way of food resources. Several of the floors may be entirely open. Many offices typically have flat roofs with access limited to a few key stairways much more suited to the constant defense of zombie warfare. Life on the roof is also more visable to passing helicopters.
If the outbreak occurred during work hours, the offices will be overrun and everyone inside will either be dead, zombified, or completely insane.
If one is lucky, a zombie attack may occur hours after work is done; giving time to arm oneself.
A bandit attack on an office may be more common.
There may be food inside an office, dependant on whoever's in the building.
One may want to store weapons in their part of the block.