Well, Taric is far cheaper (and very effective) to pick up when it comes to cost, so it isn't particularly surprising that he's a lot more common. But yes, I will admit that while Nami isn't by any stretch bad, there's just champions such as Taric and Sona that completely outdo her at the jack-of-all-trades thing she has going for her. But I also understand that buffing her CC duration and such is hard without making her run around with a 10 second cooldown Crescendo. First of all, I'd love to make her Tidecaller's Blessing/E far more visible so that her lane partner can utilize it more. Her passive, Celerity, also feels very weak, at least later on in the game. Maybe make it scale a bit with levels so that it can feel worthwhile to think about? Right now, you don't pop her skills for X effect AND Celerity. It also feels very hard to apply practically without E ready, because everything else is either better cast on the enemy (Aqua Prison, Tidal Wave) and Ebb and Flow might apply Celerity to someone who doesn't really need the speed boost.
As for the rest of her toolkit, Aqua Prison and Tidal Wave can be chained to cause some nice lockdown, even if you lose out a bit on the slow. I guess this is the reason why Aqua Prison's stun is so weak, and won't be improved. Taric only has one hard CC skill that amounts to 1.5 sec, to keep the comparisons going. I guess Ebb and Flow could have its bouncing restrictions lifted, so her sustain would improve during the laning phase and whatnot. Anyway, just a bunch of ideas on how to improve her, mainly because movement speed increases feel dumb. :x