As a supplement, not a substitute, to hand signals I think the only negative is the extra complexity each bit of equipment adds to ones bicycle. The positive is slim and may be outweighed by that negative.

The other potential slight positive is that the signal can be applied continuously vs. hand signals, such as 100' before and thru a turn or while waiting stopped at a light preparing for a left when no cyclist I know is going to stick their arm out for entire the 2min wait. It may help communiation in a few cases when the motorist can see the direction and you would otherwise not be hand signalling.

Al