Don't get me wrong, i'd love to see a mercury-logia in the team
It can't do imba things as freeze, blast or melt everything, but it'd easely beat the opponent by just touching the fella and poison him lol. :p
It'd make for a lot of fine attacks too since it's:
liquid (atacks like Akainu, and possibilitie of sneaking)
heavy (normal ppl would prlly bust their hand by just hitting it)
metal (bullet-like attacks)
toxic (floods ftw)
And u're right, he'd hardly overshadow any of the imba crew-mebers we have now.
plus most of the pirates in the new world are strong and there will be many haki-users too, so as an in-training not too imba logia, he'd fit in nicely, and wouldn't be too hax.