I've always equated sucking with being taken away to early from their mum, whether this is true on not I don't know. I think some cats are emotionally immature and that's why they do it, I think in cat psychotherapy circles it's seen as a behavior problem. Though I think it's cute and wouldn't see it that way
My mums cat Doris used to suck all the time up to the age of about 5 years old, when she eventually stopped. She completely ruined a teddy I had though and literally sucked it till it fell apart