They should be useful for long rides in the very cold, when suitable gloves (multiple layers) would become too thick to operate any switch or key and provide no feedback applying the brakes.
I already have warm 3 finger gloves like
http://www.louis.de/index.php?topic=artnr_gr&artnr_gr=201413and can add silk undergloves. But then my hand is to unmobile to add plastic overgloves too.
I do see the security concern.
But something like a "boxing glove" or a subcooled hand isn't secure either.
Others (who already use them for many years) are fond of them.
I have recently bought similar mittens and will try them when weather demands.
@Vivo
I don't think you would need them in your area, unless your ocean starts to freeze.
Our baltic sea can freeze completely so that you could theoretically walk 1500km from Germany to Finland or Russia over marine ice.