One argument against them is that in a rapid deceleration (crash) they give the wearers head more momentum due to their rather weighty construction, and are therefore increasing the risk of injury.
On the other hand, in a really rapid deceleration, it's likely your head will snap forward hard enough to hit something regardless of what you're wearing, and then a helmet is undoubtedly a good idea!
I know the RAF use them in the Grob 115 Tutor, similar to most GA types, but I've always assumed that's to acclimatise the pilots to wearing helmets in fast jets later. Odd, considering that the majority will end up on choppers or multis though...
Similar vein to the recurrent gloves debate:
Image is irrelevant. Flight safety must overrule.