There are quite a few posts about aero bars on this board - if you search you should find a few interesting threads...

Essentially aero bars are designed to make you go faster, and increase your speed on a flat road by about 2km/hour. They are pretty much an essential part of a timetrial bike/race

Some of the women here have them, not for speed but just because it is somewhere else to place their arms/hands on a long ride.

And yes, using them is frowned on in a pace line/when riding with others - basically because you have little or no control. Brakes are not positioned near your hands on aero bars, and if you have just popped them on your bike (that is not set it up as a TT specific bike which would mean shifting gear changers) then the gear shifters are a long way away too. So if you swerve, if someone in front of you stops etc etc, you have no way of stopping the bike, and because you are completely centred, much less control on direction.