The problem I have with him is that the damage he deals is pitiful right now with the best weapon I could find for him. In combat, Shulk will always be the guy drawing the Aggro since Riki's doing sub-1,000 HP attacks and Shulk's doing nearly double his damage. So Riki has the health to be a tank, but he just doesn't have the necessary Aggro-drawing ability. Granted, Reyn's really the only character I've found who can do that, and his damage-dealing isn't spectacular right now either, but Dunban seems to be a pretty good combo unit with both the damage dealing ability of a DPS and the aggro-drawing ability of a tank. I can see why people might use Riki, but I'm not going to.
Now the character right now I just can't understand why anyone would use is Melia. Her moves are just...weird. She's seems to be a sub-par healer, and she doesn't really seem to have much direct-damage capability. If Riki seems built for De-Buffing, Melia seems primarily built for Buffing. The problem is that I need a healer in that slot and Sharla is a much better healer right now.
Regarding Riki, he DOES start out weak, and even in the later game his auto-attack isn't enough to draw aggro, which is why he's not as good in the AI's hand. But his talent arts draw lots of aggro and, once upgraded, they have really low cooldown, which is why he's a great tank in the player's hands; he's death by a thousand slices, yes, but you can make him slice really, really fast.
As for Melia, she's different than every other character in one important way: where everyone (save Sharla) has to fill up the talent gauge before they can use it, Melia can use it anytime she has at least one element in play, even though the thing is apparently grayed out. What this means is that you'll summon a spirit(s), giving the group whatever buffs comes along with this, then hit the talent gauge and use those spirits offensively. Even better, this inflicts ether damage, not physical damage.
To see what I mean, use her as your character, use Summon Bolt, then click on the talent gauge. She'll fling the bolt at the targeted enemy, which usually does several thousand damage even in the early game. In essence, Melia's a mage, one which the game never teaches you how to use, and she's actually the best damage inflictor in the game. Even better, because using the talent gauge attack is a "grey" attack, she's incredible in chain attacks: her main damage inflictor not only builds on whatever color attack came before, but whatever follows automatically builds on top of
that. In essence, having her in your party is a guaranteed 5x damage chain attack.
Melia's only weakness is that her healing skill is quite weak: she can use it to drain health from herself using the method above (which usually gives her back half her health total), but it's too slow when used as a party buff. Only Sharla and Riki are ever effective group healers.