On a cold day, my hands will freeze on it as fast as a bike without them.
There are folks that praise hand guards during cold temps but also people like you Don saying there is no difference.
I also strongly believe that handguards will not prevent cold fingers/hands at all - but I have no robust evidence. If you want to keep something warm (or cold) you have to establish a layer of insulating air. Gloves do the trick - most winter gloves have an air tight layer inside just underneath the top surface.
Handguards do not do this. They might prevent the direct blast, but there‘s so much turbulence behind that any thin layer of warmer air will be dissipated in not time.
I do have the fantasy that without guards, the direct air stream might be more intensive thus cooling down the gaps between the fingers a bit more „effectively“. You might even be able to measure a very subtle but steeper heat gradient on your hand surface in some areas - but in real life your hands will get cold with our without in about the same amount of time.
Only motorcycle muffs will help preventing this as there is almost zero air movement around the gloves and this helps for sure! If it‘s really cold, put your hands (with gloves on
) in air-tight plastic bags! You‘ll be surprised how much that helps!
Positive aspects:
In rain, guards will keep the glove dry for a few minutes longer (maybe 15mins?!?) PLUS guards prevent insects from directly flying up your sleeves.