They're similar to foot stomps. An unskilled, cheap, gimmicky move that provides a high chance of injury while offering very little in entertainment value.
If a move like this is legal, might as well add hair pulling, eye pokes, groin strikes, head butting, oil checks and dick grabbing. Same basic principles.
People seem to forget that this is SPORT FIGHTING. Not Vale Tudo, not UFC 1, not convict fights...rules are meant to enhance entertainment value while keeping the product (fighters) healthy. Nothing about oblique kicks does either of those and we'll see them banned when a cash cow like McGregor gets his knee shredded.