We have them on our tandem for time trials. I don't like using them, however other than for TT's or training for TT's.

Properly set up to be aero, they're not terribly comfortable for most people.

You don't see as well around you in that position. That isn't an issue for a TT, but not the best setup for everyday riding with traffic.

You definitely do not want to use them in a paceline or group.

You loose a hand position on top of the bars. (unless you use pop up type pads, which have other issues)

You can't handle the bike as well.

And, you can deal with numb palms other ways (changing hand positions, bike fitting, learning not to cock your wrists, proper fitting gloves, conditioning, etc.)