When deciding antenna types, why might the use of an omnidirectional antenna be inadvisable?

Omnidirectional antennas are inadvisable to be used because when oriented vertically they radiate radio power equally in all horizontal directions, but heir radio power varies with the angle to the axis, reducing to zero at the axis.

Explanation:

Omnidirectional antennas radiates equal radio power in all directions perpendicular to an axis, with power varying with the elevation angle. This means that their power declines to zero on the axis. This is unlike the isotropic antennas that radiates equal power in all directions, without any reduction in radio power with angle to the axis. Omnidirectional antennas are mostly used for radio broadcasting antennas in mobile devices that use radio such as cell phones, FM radios, walkie-talkies, wireless computer networks etc.